diff --git a/go/vendor/golang.org/x/text/language/match.go b/go/vendor/golang.org/x/text/language/match.go
new file mode 100644
index 0000000..15b74d1
--- /dev/null
+++ b/go/vendor/golang.org/x/text/language/match.go
@@ -0,0 +1,933 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package language
+
+import "errors"
+
+// A MatchOption configures a Matcher.
+type MatchOption func(*matcher)
+
+// PreferSameScript will, in the absence of a match, result in the first
+// preferred tag with the same script as a supported tag to match this supported
+// tag. The default is currently true, but this may change in the future.
+func PreferSameScript(preferSame bool) MatchOption {
+	return func(m *matcher) { m.preferSameScript = preferSame }
+}
+
+// TODO(v1.0.0): consider making Matcher a concrete type, instead of interface.
+// There doesn't seem to be too much need for multiple types.
+// Making it a concrete type allows MatchStrings to be a method, which will
+// improve its discoverability.
+
+// MatchStrings parses and matches the given strings until one of them matches
+// the language in the Matcher. A string may be an Accept-Language header as
+// handled by ParseAcceptLanguage. The default language is returned if no
+// other language matched.
+func MatchStrings(m Matcher, lang ...string) (tag Tag, index int) {
+	for _, accept := range lang {
+		desired, _, err := ParseAcceptLanguage(accept)
+		if err != nil {
+			continue
+		}
+		if tag, index, conf := m.Match(desired...); conf != No {
+			return tag, index
+		}
+	}
+	tag, index, _ = m.Match()
+	return
+}
+
+// Matcher is the interface that wraps the Match method.
+//
+// Match returns the best match for any of the given tags, along with
+// a unique index associated with the returned tag and a confidence
+// score.
+type Matcher interface {
+	Match(t ...Tag) (tag Tag, index int, c Confidence)
+}
+
+// Comprehends reports the confidence score for a speaker of a given language
+// to being able to comprehend the written form of an alternative language.
+func Comprehends(speaker, alternative Tag) Confidence {
+	_, _, c := NewMatcher([]Tag{alternative}).Match(speaker)
+	return c
+}
+
+// NewMatcher returns a Matcher that matches an ordered list of preferred tags
+// against a list of supported tags based on written intelligibility, closeness
+// of dialect, equivalence of subtags and various other rules. It is initialized
+// with the list of supported tags. The first element is used as the default
+// value in case no match is found.
+//
+// Its Match method matches the first of the given Tags to reach a certain
+// confidence threshold. The tags passed to Match should therefore be specified
+// in order of preference. Extensions are ignored for matching.
+//
+// The index returned by the Match method corresponds to the index of the
+// matched tag in t, but is augmented with the Unicode extension ('u')of the
+// corresponding preferred tag. This allows user locale options to be passed
+// transparently.
+func NewMatcher(t []Tag, options ...MatchOption) Matcher {
+	return newMatcher(t, options)
+}
+
+func (m *matcher) Match(want ...Tag) (t Tag, index int, c Confidence) {
+	match, w, c := m.getBest(want...)
+	if match != nil {
+		t, index = match.tag, match.index
+	} else {
+		// TODO: this should be an option
+		t = m.default_.tag
+		if m.preferSameScript {
+		outer:
+			for _, w := range want {
+				script, _ := w.Script()
+				if script.scriptID == 0 {
+					// Don't do anything if there is no script, such as with
+					// private subtags.
+					continue
+				}
+				for i, h := range m.supported {
+					if script.scriptID == h.maxScript {
+						t, index = h.tag, i
+						break outer
+					}
+				}
+			}
+		}
+		// TODO: select first language tag based on script.
+	}
+	if w.region != 0 && t.region != 0 && t.region.contains(w.region) {
+		t, _ = Raw.Compose(t, Region{w.region})
+	}
+	// Copy options from the user-provided tag into the result tag. This is hard
+	// to do after the fact, so we do it here.
+	// TODO: add in alternative variants to -u-va-.
+	// TODO: add preferred region to -u-rg-.
+	if e := w.Extensions(); len(e) > 0 {
+		t, _ = Raw.Compose(t, e)
+	}
+	return t, index, c
+}
+
+type scriptRegionFlags uint8
+
+const (
+	isList = 1 << iota
+	scriptInFrom
+	regionInFrom
+)
+
+func (t *Tag) setUndefinedLang(id langID) {
+	if t.lang == 0 {
+		t.lang = id
+	}
+}
+
+func (t *Tag) setUndefinedScript(id scriptID) {
+	if t.script == 0 {
+		t.script = id
+	}
+}
+
+func (t *Tag) setUndefinedRegion(id regionID) {
+	if t.region == 0 || t.region.contains(id) {
+		t.region = id
+	}
+}
+
+// ErrMissingLikelyTagsData indicates no information was available
+// to compute likely values of missing tags.
+var ErrMissingLikelyTagsData = errors.New("missing likely tags data")
+
+// addLikelySubtags sets subtags to their most likely value, given the locale.
+// In most cases this means setting fields for unknown values, but in some
+// cases it may alter a value.  It returns an ErrMissingLikelyTagsData error
+// if the given locale cannot be expanded.
+func (t Tag) addLikelySubtags() (Tag, error) {
+	id, err := addTags(t)
+	if err != nil {
+		return t, err
+	} else if id.equalTags(t) {
+		return t, nil
+	}
+	id.remakeString()
+	return id, nil
+}
+
+// specializeRegion attempts to specialize a group region.
+func specializeRegion(t *Tag) bool {
+	if i := regionInclusion[t.region]; i < nRegionGroups {
+		x := likelyRegionGroup[i]
+		if langID(x.lang) == t.lang && scriptID(x.script) == t.script {
+			t.region = regionID(x.region)
+		}
+		return true
+	}
+	return false
+}
+
+func addTags(t Tag) (Tag, error) {
+	// We leave private use identifiers alone.
+	if t.private() {
+		return t, nil
+	}
+	if t.script != 0 && t.region != 0 {
+		if t.lang != 0 {
+			// already fully specified
+			specializeRegion(&t)
+			return t, nil
+		}
+		// Search matches for und-script-region. Note that for these cases
+		// region will never be a group so there is no need to check for this.
+		list := likelyRegion[t.region : t.region+1]
+		if x := list[0]; x.flags&isList != 0 {
+			list = likelyRegionList[x.lang : x.lang+uint16(x.script)]
+		}
+		for _, x := range list {
+			// Deviating from the spec. See match_test.go for details.
+			if scriptID(x.script) == t.script {
+				t.setUndefinedLang(langID(x.lang))
+				return t, nil
+			}
+		}
+	}
+	if t.lang != 0 {
+		// Search matches for lang-script and lang-region, where lang != und.
+		if t.lang < langNoIndexOffset {
+			x := likelyLang[t.lang]
+			if x.flags&isList != 0 {
+				list := likelyLangList[x.region : x.region+uint16(x.script)]
+				if t.script != 0 {
+					for _, x := range list {
+						if scriptID(x.script) == t.script && x.flags&scriptInFrom != 0 {
+							t.setUndefinedRegion(regionID(x.region))
+							return t, nil
+						}
+					}
+				} else if t.region != 0 {
+					count := 0
+					goodScript := true
+					tt := t
+					for _, x := range list {
+						// We visit all entries for which the script was not
+						// defined, including the ones where the region was not
+						// defined. This allows for proper disambiguation within
+						// regions.
+						if x.flags&scriptInFrom == 0 && t.region.contains(regionID(x.region)) {
+							tt.region = regionID(x.region)
+							tt.setUndefinedScript(scriptID(x.script))
+							goodScript = goodScript && tt.script == scriptID(x.script)
+							count++
+						}
+					}
+					if count == 1 {
+						return tt, nil
+					}
+					// Even if we fail to find a unique Region, we might have
+					// an unambiguous script.
+					if goodScript {
+						t.script = tt.script
+					}
+				}
+			}
+		}
+	} else {
+		// Search matches for und-script.
+		if t.script != 0 {
+			x := likelyScript[t.script]
+			if x.region != 0 {
+				t.setUndefinedRegion(regionID(x.region))
+				t.setUndefinedLang(langID(x.lang))
+				return t, nil
+			}
+		}
+		// Search matches for und-region. If und-script-region exists, it would
+		// have been found earlier.
+		if t.region != 0 {
+			if i := regionInclusion[t.region]; i < nRegionGroups {
+				x := likelyRegionGroup[i]
+				if x.region != 0 {
+					t.setUndefinedLang(langID(x.lang))
+					t.setUndefinedScript(scriptID(x.script))
+					t.region = regionID(x.region)
+				}
+			} else {
+				x := likelyRegion[t.region]
+				if x.flags&isList != 0 {
+					x = likelyRegionList[x.lang]
+				}
+				if x.script != 0 && x.flags != scriptInFrom {
+					t.setUndefinedLang(langID(x.lang))
+					t.setUndefinedScript(scriptID(x.script))
+					return t, nil
+				}
+			}
+		}
+	}
+
+	// Search matches for lang.
+	if t.lang < langNoIndexOffset {
+		x := likelyLang[t.lang]
+		if x.flags&isList != 0 {
+			x = likelyLangList[x.region]
+		}
+		if x.region != 0 {
+			t.setUndefinedScript(scriptID(x.script))
+			t.setUndefinedRegion(regionID(x.region))
+		}
+		specializeRegion(&t)
+		if t.lang == 0 {
+			t.lang = _en // default language
+		}
+		return t, nil
+	}
+	return t, ErrMissingLikelyTagsData
+}
+
+func (t *Tag) setTagsFrom(id Tag) {
+	t.lang = id.lang
+	t.script = id.script
+	t.region = id.region
+}
+
+// minimize removes the region or script subtags from t such that
+// t.addLikelySubtags() == t.minimize().addLikelySubtags().
+func (t Tag) minimize() (Tag, error) {
+	t, err := minimizeTags(t)
+	if err != nil {
+		return t, err
+	}
+	t.remakeString()
+	return t, nil
+}
+
+// minimizeTags mimics the behavior of the ICU 51 C implementation.
+func minimizeTags(t Tag) (Tag, error) {
+	if t.equalTags(und) {
+		return t, nil
+	}
+	max, err := addTags(t)
+	if err != nil {
+		return t, err
+	}
+	for _, id := range [...]Tag{
+		{lang: t.lang},
+		{lang: t.lang, region: t.region},
+		{lang: t.lang, script: t.script},
+	} {
+		if x, err := addTags(id); err == nil && max.equalTags(x) {
+			t.setTagsFrom(id)
+			break
+		}
+	}
+	return t, nil
+}
+
+// Tag Matching
+// CLDR defines an algorithm for finding the best match between two sets of language
+// tags. The basic algorithm defines how to score a possible match and then find
+// the match with the best score
+// (see http://www.unicode.org/reports/tr35/#LanguageMatching).
+// Using scoring has several disadvantages. The scoring obfuscates the importance of
+// the various factors considered, making the algorithm harder to understand. Using
+// scoring also requires the full score to be computed for each pair of tags.
+//
+// We will use a different algorithm which aims to have the following properties:
+// - clarity on the precedence of the various selection factors, and
+// - improved performance by allowing early termination of a comparison.
+//
+// Matching algorithm (overview)
+// Input:
+//   - supported: a set of supported tags
+//   - default:   the default tag to return in case there is no match
+//   - desired:   list of desired tags, ordered by preference, starting with
+//                the most-preferred.
+//
+// Algorithm:
+//   1) Set the best match to the lowest confidence level
+//   2) For each tag in "desired":
+//     a) For each tag in "supported":
+//        1) compute the match between the two tags.
+//        2) if the match is better than the previous best match, replace it
+//           with the new match. (see next section)
+//     b) if the current best match is Exact and pin is true the result will be
+//        frozen to the language found thusfar, although better matches may
+//        still be found for the same language.
+//   3) If the best match so far is below a certain threshold, return "default".
+//
+// Ranking:
+// We use two phases to determine whether one pair of tags are a better match
+// than another pair of tags. First, we determine a rough confidence level. If the
+// levels are different, the one with the highest confidence wins.
+// Second, if the rough confidence levels are identical, we use a set of tie-breaker
+// rules.
+//
+// The confidence level of matching a pair of tags is determined by finding the
+// lowest confidence level of any matches of the corresponding subtags (the
+// result is deemed as good as its weakest link).
+// We define the following levels:
+//   Exact    - An exact match of a subtag, before adding likely subtags.
+//   MaxExact - An exact match of a subtag, after adding likely subtags.
+//              [See Note 2].
+//   High     - High level of mutual intelligibility between different subtag
+//              variants.
+//   Low      - Low level of mutual intelligibility between different subtag
+//              variants.
+//   No       - No mutual intelligibility.
+//
+// The following levels can occur for each type of subtag:
+//   Base:    Exact, MaxExact, High, Low, No
+//   Script:  Exact, MaxExact [see Note 3], Low, No
+//   Region:  Exact, MaxExact, High
+//   Variant: Exact, High
+//   Private: Exact, No
+//
+// Any result with a confidence level of Low or higher is deemed a possible match.
+// Once a desired tag matches any of the supported tags with a level of MaxExact
+// or higher, the next desired tag is not considered (see Step 2.b).
+// Note that CLDR provides languageMatching data that defines close equivalence
+// classes for base languages, scripts and regions.
+//
+// Tie-breaking
+// If we get the same confidence level for two matches, we apply a sequence of
+// tie-breaking rules. The first that succeeds defines the result. The rules are
+// applied in the following order.
+//   1) Original language was defined and was identical.
+//   2) Original region was defined and was identical.
+//   3) Distance between two maximized regions was the smallest.
+//   4) Original script was defined and was identical.
+//   5) Distance from want tag to have tag using the parent relation [see Note 5.]
+// If there is still no winner after these rules are applied, the first match
+// found wins.
+//
+// Notes:
+// [2] In practice, as matching of Exact is done in a separate phase from
+//     matching the other levels, we reuse the Exact level to mean MaxExact in
+//     the second phase. As a consequence, we only need the levels defined by
+//     the Confidence type. The MaxExact confidence level is mapped to High in
+//     the public API.
+// [3] We do not differentiate between maximized script values that were derived
+//     from suppressScript versus most likely tag data. We determined that in
+//     ranking the two, one ranks just after the other. Moreover, the two cannot
+//     occur concurrently. As a consequence, they are identical for practical
+//     purposes.
+// [4] In case of deprecated, macro-equivalents and legacy mappings, we assign
+//     the MaxExact level to allow iw vs he to still be a closer match than
+//     en-AU vs en-US, for example.
+// [5] In CLDR a locale inherits fields that are unspecified for this locale
+//     from its parent. Therefore, if a locale is a parent of another locale,
+//     it is a strong measure for closeness, especially when no other tie
+//     breaker rule applies. One could also argue it is inconsistent, for
+//     example, when pt-AO matches pt (which CLDR equates with pt-BR), even
+//     though its parent is pt-PT according to the inheritance rules.
+//
+// Implementation Details:
+// There are several performance considerations worth pointing out. Most notably,
+// we preprocess as much as possible (within reason) at the time of creation of a
+// matcher. This includes:
+//   - creating a per-language map, which includes data for the raw base language
+//     and its canonicalized variant (if applicable),
+//   - expanding entries for the equivalence classes defined in CLDR's
+//     languageMatch data.
+// The per-language map ensures that typically only a very small number of tags
+// need to be considered. The pre-expansion of canonicalized subtags and
+// equivalence classes reduces the amount of map lookups that need to be done at
+// runtime.
+
+// matcher keeps a set of supported language tags, indexed by language.
+type matcher struct {
+	default_         *haveTag
+	supported        []*haveTag
+	index            map[langID]*matchHeader
+	passSettings     bool
+	preferSameScript bool
+}
+
+// matchHeader has the lists of tags for exact matches and matches based on
+// maximized and canonicalized tags for a given language.
+type matchHeader struct {
+	haveTags []*haveTag
+	original bool
+}
+
+// haveTag holds a supported Tag and its maximized script and region. The maximized
+// or canonicalized language is not stored as it is not needed during matching.
+type haveTag struct {
+	tag Tag
+
+	// index of this tag in the original list of supported tags.
+	index int
+
+	// conf is the maximum confidence that can result from matching this haveTag.
+	// When conf < Exact this means it was inserted after applying a CLDR equivalence rule.
+	conf Confidence
+
+	// Maximized region and script.
+	maxRegion regionID
+	maxScript scriptID
+
+	// altScript may be checked as an alternative match to maxScript. If altScript
+	// matches, the confidence level for this match is Low. Theoretically there
+	// could be multiple alternative scripts. This does not occur in practice.
+	altScript scriptID
+
+	// nextMax is the index of the next haveTag with the same maximized tags.
+	nextMax uint16
+}
+
+func makeHaveTag(tag Tag, index int) (haveTag, langID) {
+	max := tag
+	if tag.lang != 0 || tag.region != 0 || tag.script != 0 {
+		max, _ = max.canonicalize(All)
+		max, _ = addTags(max)
+		max.remakeString()
+	}
+	return haveTag{tag, index, Exact, max.region, max.script, altScript(max.lang, max.script), 0}, max.lang
+}
+
+// altScript returns an alternative script that may match the given script with
+// a low confidence.  At the moment, the langMatch data allows for at most one
+// script to map to another and we rely on this to keep the code simple.
+func altScript(l langID, s scriptID) scriptID {
+	for _, alt := range matchScript {
+		// TODO: also match cases where language is not the same.
+		if (langID(alt.wantLang) == l || langID(alt.haveLang) == l) &&
+			scriptID(alt.haveScript) == s {
+			return scriptID(alt.wantScript)
+		}
+	}
+	return 0
+}
+
+// addIfNew adds a haveTag to the list of tags only if it is a unique tag.
+// Tags that have the same maximized values are linked by index.
+func (h *matchHeader) addIfNew(n haveTag, exact bool) {
+	h.original = h.original || exact
+	// Don't add new exact matches.
+	for _, v := range h.haveTags {
+		if v.tag.equalsRest(n.tag) {
+			return
+		}
+	}
+	// Allow duplicate maximized tags, but create a linked list to allow quickly
+	// comparing the equivalents and bail out.
+	for i, v := range h.haveTags {
+		if v.maxScript == n.maxScript &&
+			v.maxRegion == n.maxRegion &&
+			v.tag.variantOrPrivateTagStr() == n.tag.variantOrPrivateTagStr() {
+			for h.haveTags[i].nextMax != 0 {
+				i = int(h.haveTags[i].nextMax)
+			}
+			h.haveTags[i].nextMax = uint16(len(h.haveTags))
+			break
+		}
+	}
+	h.haveTags = append(h.haveTags, &n)
+}
+
+// header returns the matchHeader for the given language. It creates one if
+// it doesn't already exist.
+func (m *matcher) header(l langID) *matchHeader {
+	if h := m.index[l]; h != nil {
+		return h
+	}
+	h := &matchHeader{}
+	m.index[l] = h
+	return h
+}
+
+func toConf(d uint8) Confidence {
+	if d <= 10 {
+		return High
+	}
+	if d < 30 {
+		return Low
+	}
+	return No
+}
+
+// newMatcher builds an index for the given supported tags and returns it as
+// a matcher. It also expands the index by considering various equivalence classes
+// for a given tag.
+func newMatcher(supported []Tag, options []MatchOption) *matcher {
+	m := &matcher{
+		index:            make(map[langID]*matchHeader),
+		preferSameScript: true,
+	}
+	for _, o := range options {
+		o(m)
+	}
+	if len(supported) == 0 {
+		m.default_ = &haveTag{}
+		return m
+	}
+	// Add supported languages to the index. Add exact matches first to give
+	// them precedence.
+	for i, tag := range supported {
+		pair, _ := makeHaveTag(tag, i)
+		m.header(tag.lang).addIfNew(pair, true)
+		m.supported = append(m.supported, &pair)
+	}
+	m.default_ = m.header(supported[0].lang).haveTags[0]
+	// Keep these in two different loops to support the case that two equivalent
+	// languages are distinguished, such as iw and he.
+	for i, tag := range supported {
+		pair, max := makeHaveTag(tag, i)
+		if max != tag.lang {
+			m.header(max).addIfNew(pair, true)
+		}
+	}
+
+	// update is used to add indexes in the map for equivalent languages.
+	// update will only add entries to original indexes, thus not computing any
+	// transitive relations.
+	update := func(want, have uint16, conf Confidence) {
+		if hh := m.index[langID(have)]; hh != nil {
+			if !hh.original {
+				return
+			}
+			hw := m.header(langID(want))
+			for _, ht := range hh.haveTags {
+				v := *ht
+				if conf < v.conf {
+					v.conf = conf
+				}
+				v.nextMax = 0 // this value needs to be recomputed
+				if v.altScript != 0 {
+					v.altScript = altScript(langID(want), v.maxScript)
+				}
+				hw.addIfNew(v, conf == Exact && hh.original)
+			}
+		}
+	}
+
+	// Add entries for languages with mutual intelligibility as defined by CLDR's
+	// languageMatch data.
+	for _, ml := range matchLang {
+		update(ml.want, ml.have, toConf(ml.distance))
+		if !ml.oneway {
+			update(ml.have, ml.want, toConf(ml.distance))
+		}
+	}
+
+	// Add entries for possible canonicalizations. This is an optimization to
+	// ensure that only one map lookup needs to be done at runtime per desired tag.
+	// First we match deprecated equivalents. If they are perfect equivalents
+	// (their canonicalization simply substitutes a different language code, but
+	// nothing else), the match confidence is Exact, otherwise it is High.
+	for i, lm := range langAliasMap {
+		// If deprecated codes match and there is no fiddling with the script or
+		// or region, we consider it an exact match.
+		conf := Exact
+		if langAliasTypes[i] != langMacro {
+			if !isExactEquivalent(langID(lm.from)) {
+				conf = High
+			}
+			update(lm.to, lm.from, conf)
+		}
+		update(lm.from, lm.to, conf)
+	}
+	return m
+}
+
+// getBest gets the best matching tag in m for any of the given tags, taking into
+// account the order of preference of the given tags.
+func (m *matcher) getBest(want ...Tag) (got *haveTag, orig Tag, c Confidence) {
+	best := bestMatch{}
+	for i, w := range want {
+		var max Tag
+		// Check for exact match first.
+		h := m.index[w.lang]
+		if w.lang != 0 {
+			if h == nil {
+				continue
+			}
+			// Base language is defined.
+			max, _ = w.canonicalize(Legacy | Deprecated | Macro)
+			// A region that is added through canonicalization is stronger than
+			// a maximized region: set it in the original (e.g. mo -> ro-MD).
+			if w.region != max.region {
+				w.region = max.region
+			}
+			// TODO: should we do the same for scripts?
+			// See test case: en, sr, nl ; sh ; sr
+			max, _ = addTags(max)
+		} else {
+			// Base language is not defined.
+			if h != nil {
+				for i := range h.haveTags {
+					have := h.haveTags[i]
+					if have.tag.equalsRest(w) {
+						return have, w, Exact
+					}
+				}
+			}
+			if w.script == 0 && w.region == 0 {
+				// We skip all tags matching und for approximate matching, including
+				// private tags.
+				continue
+			}
+			max, _ = addTags(w)
+			if h = m.index[max.lang]; h == nil {
+				continue
+			}
+		}
+		pin := true
+		for _, t := range want[i+1:] {
+			if w.lang == t.lang {
+				pin = false
+				break
+			}
+		}
+		// Check for match based on maximized tag.
+		for i := range h.haveTags {
+			have := h.haveTags[i]
+			best.update(have, w, max.script, max.region, pin)
+			if best.conf == Exact {
+				for have.nextMax != 0 {
+					have = h.haveTags[have.nextMax]
+					best.update(have, w, max.script, max.region, pin)
+				}
+				return best.have, best.want, best.conf
+			}
+		}
+	}
+	if best.conf <= No {
+		if len(want) != 0 {
+			return nil, want[0], No
+		}
+		return nil, Tag{}, No
+	}
+	return best.have, best.want, best.conf
+}
+
+// bestMatch accumulates the best match so far.
+type bestMatch struct {
+	have            *haveTag
+	want            Tag
+	conf            Confidence
+	pinnedRegion    regionID
+	pinLanguage     bool
+	sameRegionGroup bool
+	// Cached results from applying tie-breaking rules.
+	origLang     bool
+	origReg      bool
+	paradigmReg  bool
+	regGroupDist uint8
+	origScript   bool
+}
+
+// update updates the existing best match if the new pair is considered to be a
+// better match. To determine if the given pair is a better match, it first
+// computes the rough confidence level. If this surpasses the current match, it
+// will replace it and update the tie-breaker rule cache. If there is a tie, it
+// proceeds with applying a series of tie-breaker rules. If there is no
+// conclusive winner after applying the tie-breaker rules, it leaves the current
+// match as the preferred match.
+//
+// If pin is true and have and tag are a strong match, it will henceforth only
+// consider matches for this language. This corresponds to the nothing that most
+// users have a strong preference for the first defined language. A user can
+// still prefer a second language over a dialect of the preferred language by
+// explicitly specifying dialects, e.g. "en, nl, en-GB". In this case pin should
+// be false.
+func (m *bestMatch) update(have *haveTag, tag Tag, maxScript scriptID, maxRegion regionID, pin bool) {
+	// Bail if the maximum attainable confidence is below that of the current best match.
+	c := have.conf
+	if c < m.conf {
+		return
+	}
+	// Don't change the language once we already have found an exact match.
+	if m.pinLanguage && tag.lang != m.want.lang {
+		return
+	}
+	// Pin the region group if we are comparing tags for the same language.
+	if tag.lang == m.want.lang && m.sameRegionGroup {
+		_, sameGroup := regionGroupDist(m.pinnedRegion, have.maxRegion, have.maxScript, m.want.lang)
+		if !sameGroup {
+			return
+		}
+	}
+	if c == Exact && have.maxScript == maxScript {
+		// If there is another language and then another entry of this language,
+		// don't pin anything, otherwise pin the language.
+		m.pinLanguage = pin
+	}
+	if have.tag.equalsRest(tag) {
+	} else if have.maxScript != maxScript {
+		// There is usually very little comprehension between different scripts.
+		// In a few cases there may still be Low comprehension. This possibility
+		// is pre-computed and stored in have.altScript.
+		if Low < m.conf || have.altScript != maxScript {
+			return
+		}
+		c = Low
+	} else if have.maxRegion != maxRegion {
+		if High < c {
+			// There is usually a small difference between languages across regions.
+			c = High
+		}
+	}
+
+	// We store the results of the computations of the tie-breaker rules along
+	// with the best match. There is no need to do the checks once we determine
+	// we have a winner, but we do still need to do the tie-breaker computations.
+	// We use "beaten" to keep track if we still need to do the checks.
+	beaten := false // true if the new pair defeats the current one.
+	if c != m.conf {
+		if c < m.conf {
+			return
+		}
+		beaten = true
+	}
+
+	// Tie-breaker rules:
+	// We prefer if the pre-maximized language was specified and identical.
+	origLang := have.tag.lang == tag.lang && tag.lang != 0
+	if !beaten && m.origLang != origLang {
+		if m.origLang {
+			return
+		}
+		beaten = true
+	}
+
+	// We prefer if the pre-maximized region was specified and identical.
+	origReg := have.tag.region == tag.region && tag.region != 0
+	if !beaten && m.origReg != origReg {
+		if m.origReg {
+			return
+		}
+		beaten = true
+	}
+
+	regGroupDist, sameGroup := regionGroupDist(have.maxRegion, maxRegion, maxScript, tag.lang)
+	if !beaten && m.regGroupDist != regGroupDist {
+		if regGroupDist > m.regGroupDist {
+			return
+		}
+		beaten = true
+	}
+
+	paradigmReg := isParadigmLocale(tag.lang, have.maxRegion)
+	if !beaten && m.paradigmReg != paradigmReg {
+		if !paradigmReg {
+			return
+		}
+		beaten = true
+	}
+
+	// Next we prefer if the pre-maximized script was specified and identical.
+	origScript := have.tag.script == tag.script && tag.script != 0
+	if !beaten && m.origScript != origScript {
+		if m.origScript {
+			return
+		}
+		beaten = true
+	}
+
+	// Update m to the newly found best match.
+	if beaten {
+		m.have = have
+		m.want = tag
+		m.conf = c
+		m.pinnedRegion = maxRegion
+		m.sameRegionGroup = sameGroup
+		m.origLang = origLang
+		m.origReg = origReg
+		m.paradigmReg = paradigmReg
+		m.origScript = origScript
+		m.regGroupDist = regGroupDist
+	}
+}
+
+func isParadigmLocale(lang langID, r regionID) bool {
+	for _, e := range paradigmLocales {
+		if langID(e[0]) == lang && (r == regionID(e[1]) || r == regionID(e[2])) {
+			return true
+		}
+	}
+	return false
+}
+
+// regionGroupDist computes the distance between two regions based on their
+// CLDR grouping.
+func regionGroupDist(a, b regionID, script scriptID, lang langID) (dist uint8, same bool) {
+	const defaultDistance = 4
+
+	aGroup := uint(regionToGroups[a]) << 1
+	bGroup := uint(regionToGroups[b]) << 1
+	for _, ri := range matchRegion {
+		if langID(ri.lang) == lang && (ri.script == 0 || scriptID(ri.script) == script) {
+			group := uint(1 << (ri.group &^ 0x80))
+			if 0x80&ri.group == 0 {
+				if aGroup&bGroup&group != 0 { // Both regions are in the group.
+					return ri.distance, ri.distance == defaultDistance
+				}
+			} else {
+				if (aGroup|bGroup)&group == 0 { // Both regions are not in the group.
+					return ri.distance, ri.distance == defaultDistance
+				}
+			}
+		}
+	}
+	return defaultDistance, true
+}
+
+func (t Tag) variants() string {
+	if t.pVariant == 0 {
+		return ""
+	}
+	return t.str[t.pVariant:t.pExt]
+}
+
+// variantOrPrivateTagStr returns variants or private use tags.
+func (t Tag) variantOrPrivateTagStr() string {
+	if t.pExt > 0 {
+		return t.str[t.pVariant:t.pExt]
+	}
+	return t.str[t.pVariant:]
+}
+
+// equalsRest compares everything except the language.
+func (a Tag) equalsRest(b Tag) bool {
+	// TODO: don't include extensions in this comparison. To do this efficiently,
+	// though, we should handle private tags separately.
+	return a.script == b.script && a.region == b.region && a.variantOrPrivateTagStr() == b.variantOrPrivateTagStr()
+}
+
+// isExactEquivalent returns true if canonicalizing the language will not alter
+// the script or region of a tag.
+func isExactEquivalent(l langID) bool {
+	for _, o := range notEquivalent {
+		if o == l {
+			return false
+		}
+	}
+	return true
+}
+
+var notEquivalent []langID
+
+func init() {
+	// Create a list of all languages for which canonicalization may alter the
+	// script or region.
+	for _, lm := range langAliasMap {
+		tag := Tag{lang: langID(lm.from)}
+		if tag, _ = tag.canonicalize(All); tag.script != 0 || tag.region != 0 {
+			notEquivalent = append(notEquivalent, langID(lm.from))
+		}
+	}
+	// Maximize undefined regions of paradigm locales.
+	for i, v := range paradigmLocales {
+		max, _ := addTags(Tag{lang: langID(v[0])})
+		if v[1] == 0 {
+			paradigmLocales[i][1] = uint16(max.region)
+		}
+		if v[2] == 0 {
+			paradigmLocales[i][2] = uint16(max.region)
+		}
+	}
+}