1 files changed, 827 insertions, 0 deletions
diff --git a/src/text/template/parse/parse.go b/src/text/template/parse/parse.go
new file mode 100644
index 0000000..d43d533
--- /dev/null
+++ b/src/text/template/parse/parse.go
@@ -0,0 +1,827 @@
+// Copyright 2011 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 parse builds parse trees for templates as defined by text/template
+// and html/template. Clients should use those packages to construct templates
+// rather than this one, which provides shared internal data structures not
+// intended for general use.
+package parse
+
+import (
+	"bytes"
+	"fmt"
+	"runtime"
+	"strconv"
+	"strings"
+)
+
+// Tree is the representation of a single parsed template.
+type Tree struct {
+	Name      string    // name of the template represented by the tree.
+	ParseName string    // name of the top-level template during parsing, for error messages.
+	Root      *ListNode // top-level root of the tree.
+	Mode      Mode      // parsing mode.
+	text      string    // text parsed to create the template (or its parent)
+	// Parsing only; cleared after parse.
+	funcs      []map[string]any
+	lex        *lexer
+	token      [3]item // three-token lookahead for parser.
+	peekCount  int
+	vars       []string // variables defined at the moment.
+	treeSet    map[string]*Tree
+	actionLine int // line of left delim starting action
+	rangeDepth int
+}
+
+// A mode value is a set of flags (or 0). Modes control parser behavior.
+type Mode uint
+
+const (
+	ParseComments Mode = 1 << iota // parse comments and add them to AST
+	SkipFuncCheck                  // do not check that functions are defined
+)
+
+// Copy returns a copy of the Tree. Any parsing state is discarded.
+func (t *Tree) Copy() *Tree {
+	if t == nil {
+		return nil
+	}
+	return &Tree{
+		Name:      t.Name,
+		ParseName: t.ParseName,
+		Root:      t.Root.CopyList(),
+		text:      t.text,
+	}
+}
+
+// Parse returns a map from template name to parse.Tree, created by parsing the
+// templates described in the argument string. The top-level template will be
+// given the specified name. If an error is encountered, parsing stops and an
+// empty map is returned with the error.
+func Parse(name, text, leftDelim, rightDelim string, funcs ...map[string]any) (map[string]*Tree, error) {
+	treeSet := make(map[string]*Tree)
+	t := New(name)
+	t.text = text
+	_, err := t.Parse(text, leftDelim, rightDelim, treeSet, funcs...)
+	return treeSet, err
+}
+
+// next returns the next token.
+func (t *Tree) next() item {
+	if t.peekCount > 0 {
+		t.peekCount--
+	} else {
+		t.token[0] = t.lex.nextItem()
+	}
+	return t.token[t.peekCount]
+}
+
+// backup backs the input stream up one token.
+func (t *Tree) backup() {
+	t.peekCount++
+}
+
+// backup2 backs the input stream up two tokens.
+// The zeroth token is already there.
+func (t *Tree) backup2(t1 item) {
+	t.token[1] = t1
+	t.peekCount = 2
+}
+
+// backup3 backs the input stream up three tokens
+// The zeroth token is already there.
+func (t *Tree) backup3(t2, t1 item) { // Reverse order: we're pushing back.
+	t.token[1] = t1
+	t.token[2] = t2
+	t.peekCount = 3
+}
+
+// peek returns but does not consume the next token.
+func (t *Tree) peek() item {
+	if t.peekCount > 0 {
+		return t.token[t.peekCount-1]
+	}
+	t.peekCount = 1
+	t.token[0] = t.lex.nextItem()
+	return t.token[0]
+}
+
+// nextNonSpace returns the next non-space token.
+func (t *Tree) nextNonSpace() (token item) {
+	for {
+		token = t.next()
+		if token.typ != itemSpace {
+			break
+		}
+	}
+	return token
+}
+
+// peekNonSpace returns but does not consume the next non-space token.
+func (t *Tree) peekNonSpace() item {
+	token := t.nextNonSpace()
+	t.backup()
+	return token
+}
+
+// Parsing.
+
+// New allocates a new parse tree with the given name.
+func New(name string, funcs ...map[string]any) *Tree {
+	return &Tree{
+		Name:  name,
+		funcs: funcs,
+	}
+}
+
+// ErrorContext returns a textual representation of the location of the node in the input text.
+// The receiver is only used when the node does not have a pointer to the tree inside,
+// which can occur in old code.
+func (t *Tree) ErrorContext(n Node) (location, context string) {
+	pos := int(n.Position())
+	tree := n.tree()
+	if tree == nil {
+		tree = t
+	}
+	text := tree.text[:pos]
+	byteNum := strings.LastIndex(text, "\n")
+	if byteNum == -1 {
+		byteNum = pos // On first line.
+	} else {
+		byteNum++ // After the newline.
+		byteNum = pos - byteNum
+	}
+	lineNum := 1 + strings.Count(text, "\n")
+	context = n.String()
+	return fmt.Sprintf("%s:%d:%d", tree.ParseName, lineNum, byteNum), context
+}
+
+// errorf formats the error and terminates processing.
+func (t *Tree) errorf(format string, args ...any) {
+	t.Root = nil
+	format = fmt.Sprintf("template: %s:%d: %s", t.ParseName, t.token[0].line, format)
+	panic(fmt.Errorf(format, args...))
+}
+
+// error terminates processing.
+func (t *Tree) error(err error) {
+	t.errorf("%s", err)
+}
+
+// expect consumes the next token and guarantees it has the required type.
+func (t *Tree) expect(expected itemType, context string) item {
+	token := t.nextNonSpace()
+	if token.typ != expected {
+		t.unexpected(token, context)
+	}
+	return token
+}
+
+// expectOneOf consumes the next token and guarantees it has one of the required types.
+func (t *Tree) expectOneOf(expected1, expected2 itemType, context string) item {
+	token := t.nextNonSpace()
+	if token.typ != expected1 && token.typ != expected2 {
+		t.unexpected(token, context)
+	}
+	return token
+}
+
+// unexpected complains about the token and terminates processing.
+func (t *Tree) unexpected(token item, context string) {
+	if token.typ == itemError {
+		extra := ""
+		if t.actionLine != 0 && t.actionLine != token.line {
+			extra = fmt.Sprintf(" in action started at %s:%d", t.ParseName, t.actionLine)
+			if strings.HasSuffix(token.val, " action") {
+				extra = extra[len(" in action"):] // avoid "action in action"
+			}
+		}
+		t.errorf("%s%s", token, extra)
+	}
+	t.errorf("unexpected %s in %s", token, context)
+}
+
+// recover is the handler that turns panics into returns from the top level of Parse.
+func (t *Tree) recover(errp *error) {
+	e := recover()
+	if e != nil {
+		if _, ok := e.(runtime.Error); ok {
+			panic(e)
+		}
+		if t != nil {
+			t.stopParse()
+		}
+		*errp = e.(error)
+	}
+}
+
+// startParse initializes the parser, using the lexer.
+func (t *Tree) startParse(funcs []map[string]any, lex *lexer, treeSet map[string]*Tree) {
+	t.Root = nil
+	t.lex = lex
+	t.vars = []string{"$"}
+	t.funcs = funcs
+	t.treeSet = treeSet
+	lex.options = lexOptions{
+		emitComment: t.Mode&ParseComments != 0,
+		breakOK:     !t.hasFunction("break"),
+		continueOK:  !t.hasFunction("continue"),
+	}
+}
+
+// stopParse terminates parsing.
+func (t *Tree) stopParse() {
+	t.lex = nil
+	t.vars = nil
+	t.funcs = nil
+	t.treeSet = nil
+}
+
+// Parse parses the template definition string to construct a representation of
+// the template for execution. If either action delimiter string is empty, the
+// default ("{{" or "}}") is used. Embedded template definitions are added to
+// the treeSet map.
+func (t *Tree) Parse(text, leftDelim, rightDelim string, treeSet map[string]*Tree, funcs ...map[string]any) (tree *Tree, err error) {
+	defer t.recover(&err)
+	t.ParseName = t.Name
+	lexer := lex(t.Name, text, leftDelim, rightDelim)
+	t.startParse(funcs, lexer, treeSet)
+	t.text = text
+	t.parse()
+	t.add()
+	t.stopParse()
+	return t, nil
+}
+
+// add adds tree to t.treeSet.
+func (t *Tree) add() {
+	tree := t.treeSet[t.Name]
+	if tree == nil || IsEmptyTree(tree.Root) {
+		t.treeSet[t.Name] = t
+		return
+	}
+	if !IsEmptyTree(t.Root) {
+		t.errorf("template: multiple definition of template %q", t.Name)
+	}
+}
+
+// IsEmptyTree reports whether this tree (node) is empty of everything but space or comments.
+func IsEmptyTree(n Node) bool {
+	switch n := n.(type) {
+	case nil:
+		return true
+	case *ActionNode:
+	case *CommentNode:
+		return true
+	case *IfNode:
+	case *ListNode:
+		for _, node := range n.Nodes {
+			if !IsEmptyTree(node) {
+				return false
+			}
+		}
+		return true
+	case *RangeNode:
+	case *TemplateNode:
+	case *TextNode:
+		return len(bytes.TrimSpace(n.Text)) == 0
+	case *WithNode:
+	default:
+		panic("unknown node: " + n.String())
+	}
+	return false
+}
+
+// parse is the top-level parser for a template, essentially the same
+// as itemList except it also parses {{define}} actions.
+// It runs to EOF.
+func (t *Tree) parse() {
+	t.Root = t.newList(t.peek().pos)
+	for t.peek().typ != itemEOF {
+		if t.peek().typ == itemLeftDelim {
+			delim := t.next()
+			if t.nextNonSpace().typ == itemDefine {
+				newT := New("definition") // name will be updated once we know it.
+				newT.text = t.text
+				newT.Mode = t.Mode
+				newT.ParseName = t.ParseName
+				newT.startParse(t.funcs, t.lex, t.treeSet)
+				newT.parseDefinition()
+				continue
+			}
+			t.backup2(delim)
+		}
+		switch n := t.textOrAction(); n.Type() {
+		case nodeEnd, nodeElse:
+			t.errorf("unexpected %s", n)
+		default:
+			t.Root.append(n)
+		}
+	}
+}
+
+// parseDefinition parses a {{define}} ...  {{end}} template definition and
+// installs the definition in t.treeSet. The "define" keyword has already
+// been scanned.
+func (t *Tree) parseDefinition() {
+	const context = "define clause"
+	name := t.expectOneOf(itemString, itemRawString, context)
+	var err error
+	t.Name, err = strconv.Unquote(name.val)
+	if err != nil {
+		t.error(err)
+	}
+	t.expect(itemRightDelim, context)
+	var end Node
+	t.Root, end = t.itemList()
+	if end.Type() != nodeEnd {
+		t.errorf("unexpected %s in %s", end, context)
+	}
+	t.add()
+	t.stopParse()
+}
+
+// itemList:
+//
+//	textOrAction*
+//
+// Terminates at {{end}} or {{else}}, returned separately.
+func (t *Tree) itemList() (list *ListNode, next Node) {
+	list = t.newList(t.peekNonSpace().pos)
+	for t.peekNonSpace().typ != itemEOF {
+		n := t.textOrAction()
+		switch n.Type() {
+		case nodeEnd, nodeElse:
+			return list, n
+		}
+		list.append(n)
+	}
+	t.errorf("unexpected EOF")
+	return
+}
+
+// textOrAction:
+//
+//	text | comment | action
+func (t *Tree) textOrAction() Node {
+	switch token := t.nextNonSpace(); token.typ {
+	case itemText:
+		return t.newText(token.pos, token.val)
+	case itemLeftDelim:
+		t.actionLine = token.line
+		defer t.clearActionLine()
+		return t.action()
+	case itemComment:
+		return t.newComment(token.pos, token.val)
+	default:
+		t.unexpected(token, "input")
+	}
+	return nil
+}
+
+func (t *Tree) clearActionLine() {
+	t.actionLine = 0
+}
+
+// Action:
+//
+//	control
+//	command ("|" command)*
+//
+// Left delim is past. Now get actions.
+// First word could be a keyword such as range.
+func (t *Tree) action() (n Node) {
+	switch token := t.nextNonSpace(); token.typ {
+	case itemBlock:
+		return t.blockControl()
+	case itemBreak:
+		return t.breakControl(token.pos, token.line)
+	case itemContinue:
+		return t.continueControl(token.pos, token.line)
+	case itemElse:
+		return t.elseControl()
+	case itemEnd:
+		return t.endControl()
+	case itemIf:
+		return t.ifControl()
+	case itemRange:
+		return t.rangeControl()
+	case itemTemplate:
+		return t.templateControl()
+	case itemWith:
+		return t.withControl()
+	}
+	t.backup()
+	token := t.peek()
+	// Do not pop variables; they persist until "end".
+	return t.newAction(token.pos, token.line, t.pipeline("command", itemRightDelim))
+}
+
+// Break:
+//
+//	{{break}}
+//
+// Break keyword is past.
+func (t *Tree) breakControl(pos Pos, line int) Node {
+	if token := t.nextNonSpace(); token.typ != itemRightDelim {
+		t.unexpected(token, "{{break}}")
+	}
+	if t.rangeDepth == 0 {
+		t.errorf("{{break}} outside {{range}}")
+	}
+	return t.newBreak(pos, line)
+}
+
+// Continue:
+//
+//	{{continue}}
+//
+// Continue keyword is past.
+func (t *Tree) continueControl(pos Pos, line int) Node {
+	if token := t.nextNonSpace(); token.typ != itemRightDelim {
+		t.unexpected(token, "{{continue}}")
+	}
+	if t.rangeDepth == 0 {
+		t.errorf("{{continue}} outside {{range}}")
+	}
+	return t.newContinue(pos, line)
+}
+
+// Pipeline:
+//
+//	declarations? command ('|' command)*
+func (t *Tree) pipeline(context string, end itemType) (pipe *PipeNode) {
+	token := t.peekNonSpace()
+	pipe = t.newPipeline(token.pos, token.line, nil)
+	// Are there declarations or assignments?
+decls:
+	if v := t.peekNonSpace(); v.typ == itemVariable {
+		t.next()
+		// Since space is a token, we need 3-token look-ahead here in the worst case:
+		// in "$x foo" we need to read "foo" (as opposed to ":=") to know that $x is an
+		// argument variable rather than a declaration. So remember the token
+		// adjacent to the variable so we can push it back if necessary.
+		tokenAfterVariable := t.peek()
+		next := t.peekNonSpace()
+		switch {
+		case next.typ == itemAssign, next.typ == itemDeclare:
+			pipe.IsAssign = next.typ == itemAssign
+			t.nextNonSpace()
+			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
+			t.vars = append(t.vars, v.val)
+		case next.typ == itemChar && next.val == ",":
+			t.nextNonSpace()
+			pipe.Decl = append(pipe.Decl, t.newVariable(v.pos, v.val))
+			t.vars = append(t.vars, v.val)
+			if context == "range" && len(pipe.Decl) < 2 {
+				switch t.peekNonSpace().typ {
+				case itemVariable, itemRightDelim, itemRightParen:
+					// second initialized variable in a range pipeline
+					goto decls
+				default:
+					t.errorf("range can only initialize variables")
+				}
+			}
+			t.errorf("too many declarations in %s", context)
+		case tokenAfterVariable.typ == itemSpace:
+			t.backup3(v, tokenAfterVariable)
+		default:
+			t.backup2(v)
+		}
+	}
+	for {
+		switch token := t.nextNonSpace(); token.typ {
+		case end:
+			// At this point, the pipeline is complete
+			t.checkPipeline(pipe, context)
+			return
+		case itemBool, itemCharConstant, itemComplex, itemDot, itemField, itemIdentifier,
+			itemNumber, itemNil, itemRawString, itemString, itemVariable, itemLeftParen:
+			t.backup()
+			pipe.append(t.command())
+		default:
+			t.unexpected(token, context)
+		}
+	}
+}
+
+func (t *Tree) checkPipeline(pipe *PipeNode, context string) {
+	// Reject empty pipelines
+	if len(pipe.Cmds) == 0 {
+		t.errorf("missing value for %s", context)
+	}
+	// Only the first command of a pipeline can start with a non executable operand
+	for i, c := range pipe.Cmds[1:] {
+		switch c.Args[0].Type() {
+		case NodeBool, NodeDot, NodeNil, NodeNumber, NodeString:
+			// With A|B|C, pipeline stage 2 is B
+			t.errorf("non executable command in pipeline stage %d", i+2)
+		}
+	}
+}
+
+func (t *Tree) parseControl(allowElseIf bool, context string) (pos Pos, line int, pipe *PipeNode, list, elseList *ListNode) {
+	defer t.popVars(len(t.vars))
+	pipe = t.pipeline(context, itemRightDelim)
+	if context == "range" {
+		t.rangeDepth++
+	}
+	var next Node
+	list, next = t.itemList()
+	if context == "range" {
+		t.rangeDepth--
+	}
+	switch next.Type() {
+	case nodeEnd: //done
+	case nodeElse:
+		if allowElseIf {
+			// Special case for "else if". If the "else" is followed immediately by an "if",
+			// the elseControl will have left the "if" token pending. Treat
+			//	{{if a}}_{{else if b}}_{{end}}
+			// as
+			//	{{if a}}_{{else}}{{if b}}_{{end}}{{end}}.
+			// To do this, parse the if as usual and stop at it {{end}}; the subsequent{{end}}
+			// is assumed. This technique works even for long if-else-if chains.
+			// TODO: Should we allow else-if in with and range?
+			if t.peek().typ == itemIf {
+				t.next() // Consume the "if" token.
+				elseList = t.newList(next.Position())
+				elseList.append(t.ifControl())
+				// Do not consume the next item - only one {{end}} required.
+				break
+			}
+		}
+		elseList, next = t.itemList()
+		if next.Type() != nodeEnd {
+			t.errorf("expected end; found %s", next)
+		}
+	}
+	return pipe.Position(), pipe.Line, pipe, list, elseList
+}
+
+// If:
+//
+//	{{if pipeline}} itemList {{end}}
+//	{{if pipeline}} itemList {{else}} itemList {{end}}
+//
+// If keyword is past.
+func (t *Tree) ifControl() Node {
+	return t.newIf(t.parseControl(true, "if"))
+}
+
+// Range:
+//
+//	{{range pipeline}} itemList {{end}}
+//	{{range pipeline}} itemList {{else}} itemList {{end}}
+//
+// Range keyword is past.
+func (t *Tree) rangeControl() Node {
+	r := t.newRange(t.parseControl(false, "range"))
+	return r
+}
+
+// With:
+//
+//	{{with pipeline}} itemList {{end}}
+//	{{with pipeline}} itemList {{else}} itemList {{end}}
+//
+// If keyword is past.
+func (t *Tree) withControl() Node {
+	return t.newWith(t.parseControl(false, "with"))
+}
+
+// End:
+//
+//	{{end}}
+//
+// End keyword is past.
+func (t *Tree) endControl() Node {
+	return t.newEnd(t.expect(itemRightDelim, "end").pos)
+}
+
+// Else:
+//
+//	{{else}}
+//
+// Else keyword is past.
+func (t *Tree) elseControl() Node {
+	// Special case for "else if".
+	peek := t.peekNonSpace()
+	if peek.typ == itemIf {
+		// We see "{{else if ... " but in effect rewrite it to {{else}}{{if ... ".
+		return t.newElse(peek.pos, peek.line)
+	}
+	token := t.expect(itemRightDelim, "else")
+	return t.newElse(token.pos, token.line)
+}
+
+// Block:
+//
+//	{{block stringValue pipeline}}
+//
+// Block keyword is past.
+// The name must be something that can evaluate to a string.
+// The pipeline is mandatory.
+func (t *Tree) blockControl() Node {
+	const context = "block clause"
+
+	token := t.nextNonSpace()
+	name := t.parseTemplateName(token, context)
+	pipe := t.pipeline(context, itemRightDelim)
+
+	block := New(name) // name will be updated once we know it.
+	block.text = t.text
+	block.Mode = t.Mode
+	block.ParseName = t.ParseName
+	block.startParse(t.funcs, t.lex, t.treeSet)
+	var end Node
+	block.Root, end = block.itemList()
+	if end.Type() != nodeEnd {
+		t.errorf("unexpected %s in %s", end, context)
+	}
+	block.add()
+	block.stopParse()
+
+	return t.newTemplate(token.pos, token.line, name, pipe)
+}
+
+// Template:
+//
+//	{{template stringValue pipeline}}
+//
+// Template keyword is past. The name must be something that can evaluate
+// to a string.
+func (t *Tree) templateControl() Node {
+	const context = "template clause"
+	token := t.nextNonSpace()
+	name := t.parseTemplateName(token, context)
+	var pipe *PipeNode
+	if t.nextNonSpace().typ != itemRightDelim {
+		t.backup()
+		// Do not pop variables; they persist until "end".
+		pipe = t.pipeline(context, itemRightDelim)
+	}
+	return t.newTemplate(token.pos, token.line, name, pipe)
+}
+
+func (t *Tree) parseTemplateName(token item, context string) (name string) {
+	switch token.typ {
+	case itemString, itemRawString:
+		s, err := strconv.Unquote(token.val)
+		if err != nil {
+			t.error(err)
+		}
+		name = s
+	default:
+		t.unexpected(token, context)
+	}
+	return
+}
+
+// command:
+//
+//	operand (space operand)*
+//
+// space-separated arguments up to a pipeline character or right delimiter.
+// we consume the pipe character but leave the right delim to terminate the action.
+func (t *Tree) command() *CommandNode {
+	cmd := t.newCommand(t.peekNonSpace().pos)
+	for {
+		t.peekNonSpace() // skip leading spaces.
+		operand := t.operand()
+		if operand != nil {
+			cmd.append(operand)
+		}
+		switch token := t.next(); token.typ {
+		case itemSpace:
+			continue
+		case itemRightDelim, itemRightParen:
+			t.backup()
+		case itemPipe:
+			// nothing here; break loop below
+		default:
+			t.unexpected(token, "operand")
+		}
+		break
+	}
+	if len(cmd.Args) == 0 {
+		t.errorf("empty command")
+	}
+	return cmd
+}
+
+// operand:
+//
+//	term .Field*
+//
+// An operand is a space-separated component of a command,
+// a term possibly followed by field accesses.
+// A nil return means the next item is not an operand.
+func (t *Tree) operand() Node {
+	node := t.term()
+	if node == nil {
+		return nil
+	}
+	if t.peek().typ == itemField {
+		chain := t.newChain(t.peek().pos, node)
+		for t.peek().typ == itemField {
+			chain.Add(t.next().val)
+		}
+		// Compatibility with original API: If the term is of type NodeField
+		// or NodeVariable, just put more fields on the original.
+		// Otherwise, keep the Chain node.
+		// Obvious parsing errors involving literal values are detected here.
+		// More complex error cases will have to be handled at execution time.
+		switch node.Type() {
+		case NodeField:
+			node = t.newField(chain.Position(), chain.String())
+		case NodeVariable:
+			node = t.newVariable(chain.Position(), chain.String())
+		case NodeBool, NodeString, NodeNumber, NodeNil, NodeDot:
+			t.errorf("unexpected . after term %q", node.String())
+		default:
+			node = chain
+		}
+	}
+	return node
+}
+
+// term:
+//
+//	literal (number, string, nil, boolean)
+//	function (identifier)
+//	.
+//	.Field
+//	$
+//	'(' pipeline ')'
+//
+// A term is a simple "expression".
+// A nil return means the next item is not a term.
+func (t *Tree) term() Node {
+	switch token := t.nextNonSpace(); token.typ {
+	case itemIdentifier:
+		checkFunc := t.Mode&SkipFuncCheck == 0
+		if checkFunc && !t.hasFunction(token.val) {
+			t.errorf("function %q not defined", token.val)
+		}
+		return NewIdentifier(token.val).SetTree(t).SetPos(token.pos)
+	case itemDot:
+		return t.newDot(token.pos)
+	case itemNil:
+		return t.newNil(token.pos)
+	case itemVariable:
+		return t.useVar(token.pos, token.val)
+	case itemField:
+		return t.newField(token.pos, token.val)
+	case itemBool:
+		return t.newBool(token.pos, token.val == "true")
+	case itemCharConstant, itemComplex, itemNumber:
+		number, err := t.newNumber(token.pos, token.val, token.typ)
+		if err != nil {
+			t.error(err)
+		}
+		return number
+	case itemLeftParen:
+		return t.pipeline("parenthesized pipeline", itemRightParen)
+	case itemString, itemRawString:
+		s, err := strconv.Unquote(token.val)
+		if err != nil {
+			t.error(err)
+		}
+		return t.newString(token.pos, token.val, s)
+	}
+	t.backup()
+	return nil
+}
+
+// hasFunction reports if a function name exists in the Tree's maps.
+func (t *Tree) hasFunction(name string) bool {
+	for _, funcMap := range t.funcs {
+		if funcMap == nil {
+			continue
+		}
+		if funcMap[name] != nil {
+			return true
+		}
+	}
+	return false
+}
+
+// popVars trims the variable list to the specified length
+func (t *Tree) popVars(n int) {
+	t.vars = t.vars[:n]
+}
+
+// useVar returns a node for a variable reference. It errors if the
+// variable is not defined.
+func (t *Tree) useVar(pos Pos, name string) Node {
+	v := t.newVariable(pos, name)
+	for _, varName := range t.vars {
+		if varName == v.Ident[0] {
+			return v
+		}
+	}
+	t.errorf("undefined variable %q", v.Ident[0])
+	return nil
+}