diff --git a/char.go b/char.go
index 71f30b3..d725b11 100644
--- a/char.go
+++ b/char.go
@@ -23,33 +23,30 @@ func newChar(
 	}
 }
 
-func (p *charParser) nodeName() string                  { return p.name }
-func (p *charParser) setNodeName(n string)              { p.name = n }
-func (p *charParser) nodeID() int                       { return p.id }
-func (p *charParser) setID(id int)                      { p.id = id }
-func (p *charParser) commitType() CommitType            { return Alias }
-func (p *charParser) setCommitType(ct CommitType)       {}
-func (p *charParser) validate(*registry, *idSet) error  { return nil }
-func (p *charParser) normalize(*registry, *idSet) error { return nil }
+func (p *charParser) nodeName() string            { return p.name }
+func (p *charParser) setNodeName(n string)        { p.name = n }
+func (p *charParser) nodeID() int                 { return p.id }
+func (p *charParser) setID(id int)                { p.id = id }
+func (p *charParser) commitType() CommitType      { return Alias }
+func (p *charParser) setCommitType(ct CommitType) {}
+func (p *charParser) validate(*registry) error    { return nil }
+func (p *charParser) init(*registry)              {}
 
-func (p *charParser) init(r *registry) error { return nil }
-
-func (p *charParser) setIncludedBy(r *registry, includedBy int, parsers *idSet) error {
-	p.includedBy = appendIfMissing(p.includedBy, includedBy)
-	return nil
-}
-
-func (p *charParser) parser(r *registry, parsers *idSet) (parser, error) {
-	if parsers.has(p.id) {
-		panic(cannotIncludeParsers(p.name))
+func (p *charParser) setIncludedBy(r *registry, includedBy int) {
+	if intsContain(p.includedBy, includedBy) {
+		return
 	}
 
+	p.includedBy = append(p.includedBy, includedBy)
+}
+
+func (p *charParser) parser(r *registry) parser {
 	if _, ok := r.parser(p.name); ok {
-		return p, nil
+		return p
 	}
 
 	r.setParser(p)
-	return p, nil
+	return p
 }
 
 func (p *charParser) builder() builder {
diff --git a/choice.go b/choice.go
index 43ec7a1..f367ca5 100644
--- a/choice.go
+++ b/choice.go
@@ -1,12 +1,14 @@
 package treerack
 
 type choiceDefinition struct {
-	name       string
-	id         int
-	commit     CommitType
-	elements   []string
-	includedBy []int
-	cbuilder   *choiceBuilder
+	name        string
+	id          int
+	commit      CommitType
+	elements    []string
+	includedBy  []int
+	cbuilder    *choiceBuilder
+	validated   bool
+	initialized bool
 }
 
 type choiceParser struct {
@@ -40,30 +42,34 @@ func (d *choiceDefinition) setID(id int)                { d.id = id }
 func (d *choiceDefinition) commitType() CommitType      { return d.commit }
 func (d *choiceDefinition) setCommitType(ct CommitType) { d.commit = ct }
 
-func (d *choiceDefinition) validate(r *registry, path *idSet) error {
+func (d *choiceDefinition) validate(r *registry) error {
+	if d.validated {
+		return nil
+	}
+
+	d.validated = true
+
 	for i := range d.elements {
-		if _, ok := r.definitions[d.elements[i]]; !ok {
+		e, ok := r.definitions[d.elements[i]]
+		if !ok {
 			return parserNotFound(d.elements[i])
 		}
+
+		if err := e.validate(r); err != nil {
+			return err
+		}
 	}
 
 	return nil
 }
 
-func (d *choiceDefinition) normalize(r *registry, path *idSet) error {
-	if path.has(d.id) {
-		return nil
+func (d *choiceDefinition) init(r *registry) {
+	if d.initialized {
+		return
 	}
 
-	path.set(d.id)
-	for i := range d.elements {
-		r.definitions[d.elements[i]].normalize(r, path)
-	}
+	d.initialized = true
 
-	return nil
-}
-
-func (d *choiceDefinition) init(r *registry) error {
 	if d.cbuilder == nil {
 		d.cbuilder = &choiceBuilder{
 			name:       d.name,
@@ -74,21 +80,19 @@ func (d *choiceDefinition) init(r *registry) error {
 	}
 
 	for _, e := range d.elements {
-		// TODO: handle undefined reference
-		d.cbuilder.elements = append(d.cbuilder.elements, r.definitions[e].builder())
+		def := r.definitions[e]
+		d.cbuilder.elements = append(d.cbuilder.elements, def.builder())
+		def.init(r)
+		def.setIncludedBy(r, d.id)
 	}
-
-	parsers := &idSet{}
-	parsers.set(d.id)
-	return setItemsIncludedBy(r, d.elements, d.id, parsers)
 }
 
-func (d *choiceDefinition) setIncludedBy(r *registry, includedBy int, parsers *idSet) error {
-	if parsers.has(d.id) {
-		return nil
+func (d *choiceDefinition) setIncludedBy(r *registry, includedBy int) {
+	if intsContain(d.includedBy, includedBy) {
+		return
 	}
 
-	d.includedBy = appendIfMissing(d.includedBy, includedBy)
+	d.includedBy = append(d.includedBy, includedBy)
 
 	if d.cbuilder == nil {
 		d.cbuilder = &choiceBuilder{
@@ -101,18 +105,19 @@ func (d *choiceDefinition) setIncludedBy(r *registry, includedBy int, parsers *i
 
 	d.cbuilder.includedBy.set(includedBy)
 
-	parsers.set(d.id)
-	return setItemsIncludedBy(r, d.elements, includedBy, parsers)
+	for _, e := range d.elements {
+		r.definitions[e].setIncludedBy(r, includedBy)
+	}
 }
 
 // TODO:
 // - it may be possible to initialize the parsers non-recursively
 // - maybe the whole definition, parser and builder can be united
 
-func (d *choiceDefinition) parser(r *registry, parsers *idSet) (parser, error) {
+func (d *choiceDefinition) parser(r *registry) parser {
 	p, ok := r.parser(d.name)
 	if ok {
-		return p, nil
+		return p
 	}
 
 	cp := &choiceParser{
@@ -125,8 +130,6 @@ func (d *choiceDefinition) parser(r *registry, parsers *idSet) (parser, error) {
 	r.setParser(cp)
 
 	var elements []parser
-	parsers.set(d.id)
-	defer parsers.unset(d.id)
 	for _, e := range d.elements {
 		element, ok := r.parser(e)
 		if ok {
@@ -134,16 +137,12 @@ func (d *choiceDefinition) parser(r *registry, parsers *idSet) (parser, error) {
 			continue
 		}
 
-		element, err := r.definitions[e].parser(r, parsers)
-		if err != nil {
-			return nil, err
-		}
-
+		element = r.definitions[e].parser(r)
 		elements = append(elements, element)
 	}
 
 	cp.elements = elements
-	return cp, nil
+	return cp
 }
 
 func (d *choiceDefinition) builder() builder {
diff --git a/parse.go b/parse.go
index 5a7d907..01a41d2 100644
--- a/parse.go
+++ b/parse.go
@@ -9,11 +9,10 @@ type definition interface {
 	commitType() CommitType
 	setCommitType(CommitType)
 	setID(int)
-	validate(*registry, *idSet) error
-	normalize(*registry, *idSet) error
-	init(*registry) error
-	setIncludedBy(*registry, int, *idSet) error
-	parser(*registry, *idSet) (parser, error)
+	validate(*registry) error
+	init(*registry)
+	setIncludedBy(*registry, int)
+	parser(*registry) parser
 	builder() builder
 }
 
@@ -47,36 +46,6 @@ func intsContain(is []int, i int) bool {
 	return false
 }
 
-func appendIfMissing(is []int, i int) []int {
-	if intsContain(is, i) {
-		return is
-	}
-
-	return append(is, i)
-}
-
-func setItemsIncludedBy(r *registry, items []string, includedBy int, parsers *idSet) error {
-	for _, item := range items {
-		di, ok := r.definition(item)
-		if !ok {
-			return ErrNoParsersDefined
-		}
-
-		di.setIncludedBy(r, includedBy, parsers)
-	}
-
-	return nil
-}
-
-func sequenceItemNames(items []SequenceItem) []string {
-	names := make([]string, len(items))
-	for i := range items {
-		names[i] = items[i].Name
-	}
-
-	return names
-}
-
 func parse(p parser, c *context) error {
 	p.parse(c)
 	if c.readErr != nil {
diff --git a/registry.go b/registry.go
index 497e348..c6f0fbd 100644
--- a/registry.go
+++ b/registry.go
@@ -8,13 +8,19 @@ type registry struct {
 	parsers     map[string]parser
 }
 
-func newRegistry() *registry {
-	return &registry{
+func newRegistry(defs ...definition) *registry {
+	r := &registry{
 		ids:         make(map[string]int),
 		names:       make(map[int]string),
 		definitions: make(map[string]definition),
 		parsers:     make(map[string]parser),
 	}
+
+	for _, def := range defs {
+		r.setDefinition(def)
+	}
+
+	return r
 }
 
 func (r *registry) definition(name string) (definition, bool) {
@@ -45,3 +51,12 @@ func (r *registry) setDefinition(d definition) error {
 func (r *registry) setParser(p parser) {
 	r.parsers[p.nodeName()] = p
 }
+
+func (r *registry) getDefinitions() []definition {
+	var defs []definition
+	for _, def := range r.definitions {
+		defs = append(defs, def)
+	}
+
+	return defs
+}
diff --git a/sequence.go b/sequence.go
index 6b7bc2e..30246ba 100644
--- a/sequence.go
+++ b/sequence.go
@@ -1,14 +1,16 @@
 package treerack
 
 type sequenceDefinition struct {
-	name       string
-	id         int
-	commit     CommitType
-	items      []SequenceItem
-	includedBy []int
-	ranges     [][]int
-	sbuilder   *sequenceBuilder
-	allChars   bool
+	name        string
+	id          int
+	commit      CommitType
+	items       []SequenceItem
+	includedBy  []int
+	ranges      [][]int
+	sbuilder    *sequenceBuilder
+	allChars    bool
+	validated   bool
+	initialized bool
 }
 
 type sequenceParser struct {
@@ -46,36 +48,22 @@ func (d *sequenceDefinition) setID(id int)                { d.id = id }
 func (d *sequenceDefinition) commitType() CommitType      { return d.commit }
 func (d *sequenceDefinition) setCommitType(ct CommitType) { d.commit = ct }
 
-func (d *sequenceDefinition) validate(r *registry, path *idSet) error {
-	for i := range d.items {
-		if _, ok := r.definition(d.items[i].Name); !ok {
-			return parserNotFound(d.items[i].Name)
-		}
-	}
-
-	return nil
-}
-
-func (d *sequenceDefinition) normalizeItems() {
-	for i := range d.items {
-		if d.items[i].Min == 0 && d.items[i].Max == 0 {
-			d.items[i].Min, d.items[i].Max = 1, 1
-		} else if d.items[i].Max == 0 {
-			d.items[i].Max = -1
-		}
-	}
-}
-
-func (d *sequenceDefinition) normalize(r *registry, path *idSet) error {
-	if path.has(d.id) {
+func (d *sequenceDefinition) validate(r *registry) error {
+	if d.validated {
 		return nil
 	}
 
-	// d.normalizeItems()
+	d.validated = true
 
-	path.set(d.id)
 	for i := range d.items {
-		r.definitions[d.items[i].Name].normalize(r, path)
+		ii, ok := r.definition(d.items[i].Name)
+		if !ok {
+			return parserNotFound(d.items[i].Name)
+		}
+
+		if err := ii.validate(r); err != nil {
+			return err
+		}
 	}
 
 	return nil
@@ -85,7 +73,21 @@ func (d *sequenceDefinition) includeItems() bool {
 	return len(d.items) == 1 && d.items[0].Min == 1 && d.items[0].Max == 1
 }
 
-func (d *sequenceDefinition) init(r *registry) error {
+func (d *sequenceDefinition) init(r *registry) {
+	if d.initialized {
+		return
+	}
+
+	d.initialized = true
+
+	for i := range d.items {
+		if d.items[i].Min == 0 && d.items[i].Max == 0 {
+			d.items[i].Min, d.items[i].Max = 1, 1
+		} else if d.items[i].Max == 0 {
+			d.items[i].Max = -1
+		}
+	}
+
 	if d.sbuilder == nil {
 		d.sbuilder = &sequenceBuilder{
 			name:       d.name,
@@ -113,6 +115,8 @@ func (d *sequenceDefinition) init(r *registry) error {
 				allChars = false
 			}
 		}
+
+		def.init(r)
 	}
 
 	d.sbuilder.ranges = d.ranges
@@ -120,20 +124,18 @@ func (d *sequenceDefinition) init(r *registry) error {
 	d.allChars = allChars
 
 	if !d.includeItems() {
-		return nil
+		return
 	}
 
-	parsers := &idSet{}
-	parsers.set(d.id)
-	return setItemsIncludedBy(r, sequenceItemNames(d.items), d.id, parsers)
+	r.definitions[d.items[0].Name].setIncludedBy(r, d.id)
 }
 
-func (d *sequenceDefinition) setIncludedBy(r *registry, includedBy int, parsers *idSet) error {
-	if parsers.has(d.id) {
-		return nil
+func (d *sequenceDefinition) setIncludedBy(r *registry, includedBy int) {
+	if intsContain(d.includedBy, includedBy) {
+		return
 	}
 
-	d.includedBy = appendIfMissing(d.includedBy, includedBy)
+	d.includedBy = append(d.includedBy, includedBy)
 
 	if d.sbuilder == nil {
 		d.sbuilder = &sequenceBuilder{
@@ -147,21 +149,16 @@ func (d *sequenceDefinition) setIncludedBy(r *registry, includedBy int, parsers
 	d.sbuilder.includedBy.set(includedBy)
 
 	if !d.includeItems() {
-		return nil
+		return
 	}
 
-	parsers.set(d.id)
-	return setItemsIncludedBy(r, sequenceItemNames(d.items), includedBy, parsers)
+	r.definitions[d.items[0].Name].setIncludedBy(r, includedBy)
 }
 
-func (d *sequenceDefinition) parser(r *registry, parsers *idSet) (parser, error) {
-	if parsers.has(d.id) {
-		panic(cannotIncludeParsers(d.name))
-	}
-
+func (d *sequenceDefinition) parser(r *registry) parser {
 	p, ok := r.parser(d.name)
 	if ok {
-		return p, nil
+		return p
 	}
 
 	sp := &sequenceParser{
@@ -175,8 +172,6 @@ func (d *sequenceDefinition) parser(r *registry, parsers *idSet) (parser, error)
 	r.setParser(sp)
 
 	var items []parser
-	parsers.set(d.id)
-	defer parsers.unset(d.id)
 	for _, item := range d.items {
 		pi, ok := r.parser(item.Name)
 		if ok {
@@ -184,17 +179,13 @@ func (d *sequenceDefinition) parser(r *registry, parsers *idSet) (parser, error)
 			continue
 		}
 
-		pi, err := r.definitions[item.Name].parser(r, parsers)
-		if err != nil {
-			return nil, err
-		}
-
+		pi = r.definitions[item.Name].parser(r)
 		items = append(items, pi)
 	}
 
 	sp.items = items
 	sp.ranges = d.ranges
-	return sp, nil
+	return sp
 }
 
 func (d *sequenceDefinition) builder() builder {
diff --git a/syntax.go b/syntax.go
index 4e6657c..10dbf89 100644
--- a/syntax.go
+++ b/syntax.go
@@ -234,35 +234,22 @@ func (s *Syntax) Init() error {
 		return ErrRootWhitespace
 	}
 
-	s.registry = initWhitespace(s.registry)
+	defs := s.registry.getDefinitions()
 
-	for _, def := range s.registry.definitions {
-		if def.commitType()&Root != 0 {
-			s.root = def
-			break
-		}
+	if hasWhitespace(defs) {
+		defs, s.root = applyWhitespace(defs)
+		s.registry = newRegistry(defs...)
 	}
 
-	if err := s.root.validate(s.registry, &idSet{}); err != nil {
-		return err
-	}
-
-	if err := s.root.normalize(s.registry, &idSet{}); err != nil {
-		return err
-	}
-
-	for _, p := range s.registry.definitions {
-		p.init(s.registry)
-	}
-
-	var err error
-	s.parser, err = s.root.parser(s.registry, &idSet{})
-	if err != nil {
+	if err := s.root.validate(s.registry); err != nil {
 		s.initFailed = true
 		return err
 	}
 
+	s.root.init(s.registry)
+	s.parser = s.root.parser(s.registry)
 	s.builder = s.root.builder()
+
 	s.initialized = true
 	return nil
 }
diff --git a/whitespace.go b/whitespace.go
index ff00779..efd8c82 100644
--- a/whitespace.go
+++ b/whitespace.go
@@ -12,9 +12,9 @@ func brokenRegistryError(err error) error {
 	return fmt.Errorf("broken registry: %v", err)
 }
 
-func splitWhitespaceDefs(all map[string]definition) ([]definition, []definition) {
+func splitWhitespaceDefs(defs []definition) ([]definition, []definition) {
 	var whitespaceDefs, nonWhitespaceDefs []definition
-	for _, def := range all {
+	for _, def := range defs {
 		if def.commitType()&Whitespace != 0 {
 			def.setCommitType(def.commitType() | Alias)
 			whitespaceDefs = append(whitespaceDefs, def)
@@ -54,14 +54,10 @@ func mergeWhitespaceDefs(ws []definition) definition {
 	return newChoice(whitespaceName, Alias, names)
 }
 
-// TODO: validate min and max
-
 func patchName(s ...string) string {
 	return strings.Join(s, ":")
 }
 
-// TODO: check what's more useful: update quantified char classes or not
-
 func applyWhitespaceToSeq(s *sequenceDefinition) []definition {
 	var (
 		defs  []definition
@@ -113,7 +109,7 @@ func applyWhitespaceToSeq(s *sequenceDefinition) []definition {
 	return defs
 }
 
-func applyWhitespace(defs []definition) []definition {
+func applyWhitespaceToDefs(defs []definition) []definition {
 	var defsWS []definition
 	for _, def := range defs {
 		if def.commitType()&NoWhitespace != 0 {
@@ -133,7 +129,7 @@ func applyWhitespace(defs []definition) []definition {
 	return defsWS
 }
 
-func applyWhitespaceRoot(root definition) (definition, definition) {
+func applyWhitespaceToRoot(root definition) (definition, definition) {
 	original, name := root, root.nodeName()
 	wsName := patchName(name, "wsroot")
 
@@ -158,30 +154,34 @@ func applyWhitespaceRoot(root definition) (definition, definition) {
 	return original, root
 }
 
-func registerPatched(r *registry, defs ...definition) {
-	for _, def := range defs {
-		if err := r.setDefinition(def); err != nil {
-			panic(brokenRegistryError(err))
+func hasWhitespace(defs []definition) bool {
+	for i := range defs {
+		if defs[i].commitType()&Whitespace != 0 {
+			return true
 		}
 	}
+
+	return false
 }
 
-func initWhitespace(r *registry) *registry {
-	whitespaceDefs, defs := splitWhitespaceDefs(r.definitions)
-	if len(whitespaceDefs) == 0 {
-		return r
-	}
-
+func applyWhitespace(defs []definition) ([]definition, definition) {
+	whitespaceDefs, defs := splitWhitespaceDefs(defs)
 	whitespace := mergeWhitespaceDefs(whitespaceDefs)
-	defs = applyWhitespace(defs)
+
+	defs = applyWhitespaceToDefs(defs)
 
 	root, defs := splitRoot(defs)
-	originalRoot, root := applyWhitespaceRoot(root)
+	originalRoot, root := applyWhitespaceToRoot(root)
 
-	r = newRegistry()
-	registerPatched(r, whitespace)
-	registerPatched(r, whitespaceDefs...)
-	registerPatched(r, defs...)
-	registerPatched(r, originalRoot, root)
-	return r
+	defs = append(
+		append(
+			defs,
+			whitespaceDefs...,
+		),
+		whitespace,
+		originalRoot,
+		root,
+	)
+
+	return defs, root
 }