diff --git a/chunks/ast.go b/chunks/ast.go
index f8ee100..90ad1b8 100644
--- a/chunks/ast.go
+++ b/chunks/ast.go
@@ -37,7 +37,7 @@ type ASTObject struct {
 // ASTControlTag is a control tag.
 type ASTControlTag struct {
 	Chunk
-	cd       *ControlTagDefinition
+	cd       *controlTagDefinition
 	Body     []ASTNode
 	Branches []*ASTControlTag
 }
diff --git a/chunks/control_tags.go b/chunks/control_tags.go
index 7e902db..e7f4f41 100644
--- a/chunks/control_tags.go
+++ b/chunks/control_tags.go
@@ -5,80 +5,81 @@ import (
 	"io"
 )
 
-// ControlTagDefinitions is a map of tag names to control tag definitions.
-var ControlTagDefinitions = map[string]*ControlTagDefinition{}
+// controlTagDefinitions is a map of tag names to control tag definitions.
+var controlTagDefinitions = map[string]*controlTagDefinition{}
 
 // ControlTagAction runs the interpreter.
 type ControlTagAction func(ASTControlTag) func(io.Writer, Context) error
 
-// ControlTagDefinition tells the parser how to parse control tags.
-type ControlTagDefinition struct {
-	Name        string
-	IsBranchTag bool
-	IsEndTag    bool
-	SyntaxModel *ControlTagDefinition
-	Parent      *ControlTagDefinition
+// controlTagDefinition tells the parser how to parse control tags.
+type controlTagDefinition struct {
+	name        string
+	isBranchTag, isEndTag bool
+	syntaxModel *controlTagDefinition
+	parent      *controlTagDefinition
 	action      ControlTagAction
 }
 
-func (c *ControlTagDefinition) CompatibleParent(p *ControlTagDefinition) bool {
+func (c *controlTagDefinition) compatibleParent(p *controlTagDefinition) bool {
 	if p == nil {
 		return false
 	}
-	if p.SyntaxModel != nil {
-		p = p.SyntaxModel
+	if p.syntaxModel != nil {
+		p = p.syntaxModel
 	}
-	return c.Parent == p
+	return c.parent == p
 }
 
-func (c *ControlTagDefinition) RequiresParent() bool {
-	return c.IsBranchTag || c.IsEndTag
+func (c *controlTagDefinition) requiresParent() bool {
+	return c.isBranchTag || c.isEndTag
 }
 
-func (c *ControlTagDefinition) IsStartTag() bool {
-	return !c.IsBranchTag && !c.IsEndTag
+func (c *controlTagDefinition) isStartTag() bool {
+	return !c.isBranchTag && !c.isEndTag
 }
 
-// DefineControlTag defines a control tag and its matching end tag.
-func DefineControlTag(name string) *ControlTagDefinition {
-	ct := &ControlTagDefinition{Name: name}
-	addControlTagDefinition(ct)
-	addControlTagDefinition(&ControlTagDefinition{Name: "end" + name, IsEndTag: true, Parent: ct})
-	return ct
+func addControlTagDefinition(ct *controlTagDefinition) {
+	controlTagDefinitions[ct.name] = ct
 }
 
-func FindControlDefinition(name string) (*ControlTagDefinition, bool) {
-	ct, found := ControlTagDefinitions[name]
+func findControlTagDefinition(name string) (*controlTagDefinition, bool) {
+	ct, found := controlTagDefinitions[name]
 	return ct, found
 }
 
-func addControlTagDefinition(ct *ControlTagDefinition) {
-	ControlTagDefinitions[ct.Name] = ct
+type tagBuilder struct {tag *controlTagDefinition}
+
+// DefineControlTag defines a control tag and its matching end tag.
+func DefineControlTag(name string) tagBuilder {
+	ct := &controlTagDefinition{name: name}
+	addControlTagDefinition(ct)
+	addControlTagDefinition(&controlTagDefinition{name: "end" + name, isEndTag: true, parent: ct})
+	return tagBuilder{ct}
 }
 
 // Branch tells the parser that the named tag can appear immediately between this tag and its end tag,
 // so long as it is not nested within any other control tags.
-func (ct *ControlTagDefinition) Branch(name string) *ControlTagDefinition {
-	addControlTagDefinition(&ControlTagDefinition{Name: name, IsBranchTag: true, Parent: ct})
-	return ct
+func (b tagBuilder) Branch(name string) tagBuilder {
+	addControlTagDefinition(&controlTagDefinition{name: name, isBranchTag: true, parent: b.tag})
+	return b
 }
 
 // Governs tells the parser that the tags can appear anywhere between this tag and its end tag.
-func (ct *ControlTagDefinition) Governs(_ []string) *ControlTagDefinition {
-	return ct
+func (b tagBuilder) Governs(_ []string) tagBuilder {
+	return b
 }
 
 // SameSyntaxAs tells the parser that this tag has the same syntax as the named tag.
-func (ct *ControlTagDefinition) SameSyntaxAs(name string) *ControlTagDefinition {
-	ot := ControlTagDefinitions[name]
+func (b tagBuilder) SameSyntaxAs(name string) tagBuilder {
+	ot := controlTagDefinitions[name]
 	if ot == nil {
 		panic(fmt.Errorf("undefined: %s", name))
 	}
-	ct.SyntaxModel = ot
-	return ct
+	b.tag.syntaxModel = ot
+	return b
 }
 
 // Action sets the action for a control tag definition.
-func (ct *ControlTagDefinition) Action(fn ControlTagAction) {
-	ct.action = fn
+func (b tagBuilder) Action(fn ControlTagAction) {
+	b.tag.action = fn
 }
diff --git a/chunks/marshal.go b/chunks/marshal.go
index 8496d17..2f888f0 100644
--- a/chunks/marshal.go
+++ b/chunks/marshal.go
@@ -37,7 +37,7 @@ func (n ASTChunks) MarshalYAML() (interface{}, error) {
 // MarshalYAML marshalls a chunk for debugging.
 func (n ASTControlTag) MarshalYAML() (interface{}, error) {
 	return map[string]map[string]interface{}{
-		n.cd.Name: {
+		n.cd.name: {
 			"args":     n.Args,
 			"body":     n.Body,
 			"branches": n.Branches,
diff --git a/chunks/parser.go b/chunks/parser.go
index be1366d..2c4ad90 100644
--- a/chunks/parser.go
+++ b/chunks/parser.go
@@ -7,14 +7,14 @@ import (
 // Parse creates an AST from a sequence of Chunks.
 func Parse(chunks []Chunk) (ASTNode, error) {
 	type frame struct {
-		cd *ControlTagDefinition
+		cd *controlTagDefinition
 		cn *ASTControlTag
 		ap *[]ASTNode
 	}
 	var (
 		root  = &ASTSeq{}
 		ap    = &root.Children // pointer to current node accumulation slice
-		ccd   *ControlTagDefinition
+		ccd   *controlTagDefinition
 		ccn   *ASTControlTag
 		stack []frame // stack of control structures
 	)
@@ -25,24 +25,24 @@ func Parse(chunks []Chunk) (ASTNode, error) {
 		case TextChunkType:
 			*ap = append(*ap, &ASTText{Chunk: c})
 		case TagChunkType:
-			if cd, ok := FindControlDefinition(c.Tag); ok {
+			if cd, ok := findControlTagDefinition(c.Tag); ok {
 				switch {
-				case cd.RequiresParent() && !cd.CompatibleParent(ccd):
+				case cd.requiresParent() && !cd.compatibleParent(ccd):
 					suffix := ""
 					if ccd != nil {
-						suffix = "; immediate parent is " + ccd.Name
+						suffix = "; immediate parent is " + ccd.name
 					}
-					return nil, fmt.Errorf("%s not inside %s%s", cd.Name, cd.Parent.Name, suffix)
-				case cd.IsStartTag():
+					return nil, fmt.Errorf("%s not inside %s%s", cd.name, cd.parent.name, suffix)
+				case cd.isStartTag():
 					stack = append(stack, frame{cd: ccd, cn: ccn, ap: ap})
 					ccd, ccn = cd, &ASTControlTag{Chunk: c, cd: cd}
 					*ap = append(*ap, ccn)
 					ap = &ccn.Body
-				case cd.IsBranchTag:
+				case cd.isBranchTag:
 					n := &ASTControlTag{Chunk: c, cd: cd}
 					ccn.Branches = append(ccn.Branches, n)
 					ap = &n.Body
-				case cd.IsEndTag:
+				case cd.isEndTag:
 					f := stack[len(stack)-1]
 					ccd, ccn, ap, stack = f.cd, f.cn, f.ap, stack[:len(stack)-1]
 				}
@@ -68,7 +68,7 @@ func Parse(chunks []Chunk) (ASTNode, error) {
 		}
 	}
 	if ccd != nil {
-		return nil, fmt.Errorf("unterminated %s tag", ccd.Name)
+		return nil, fmt.Errorf("unterminated %s tag", ccd.name)
 	}
 	if len(root.Children) == 1 {
 		return root.Children[0], nil
diff --git a/chunks/render.go b/chunks/render.go
index ba45893..c248577 100644
--- a/chunks/render.go
+++ b/chunks/render.go
@@ -45,7 +45,7 @@ func (ctx Context) RenderASTSequence(w io.Writer, seq []ASTNode) error {
 
 // Render evaluates an AST node and writes the result to an io.Writer.
 func (n *ASTControlTag) Render(w io.Writer, ctx Context) error {
-	cd, ok := FindControlDefinition(n.Tag)
+	cd, ok := findControlTagDefinition(n.Tag)
 	if !ok || cd.action == nil {
 		return fmt.Errorf("unimplemented tag: %s", n.Tag)
 	}