avoid panic'ing in `used but not defined' cases

gistfile1.diff

diff --git a/peg.go b/peg.go
--- a/peg.go
+++ b/peg.go
@@ -36,6 +36,7 @@
  TypeStar
  TypePlus
  TypePeg
+ TypeNil
  TypeLast
 )
 
@@ -75,6 +76,7 @@
 }
 
 func (r *rule) GetExpression() Node {
+ if r.expression == nil {return nilNode}
  return r.expression
 }
 
@@ -86,7 +88,7 @@
  return r.name
 }
 
-/* Used to represent TypeName, TypeDot, TypeCharacter, TypeString, TypeClass, and TypePredicate. */
+/* Used to represent TypeName, TypeDot, TypeCharacter, TypeString, TypeClass, TypePredicate, and TypeNil. */
 type Token interface {
  Node
  GetClass() *characterClass
@@ -106,6 +108,8 @@
  return t.string
 }
 
+var nilNode = &token{Type: TypeNil, string: "<nil>"}
+
 /* Used to represent TypeAction. */
 type Action interface {
  Node
@@ -298,7 +302,10 @@
  t.PushBack(rule)
 }
 
-func (t *Tree) AddName(text string) {t.push(&token{Type: TypeName, string: text})}
+func (t *Tree) AddName(text string) {
+ t.rules[text] = &rule{}
+ t.push(&token{Type: TypeName, string: text})
+}
 var dot *token = &token{Type: TypeDot, string: "."}
 func (t *Tree) AddDot() {t.push(dot)}
 func (t *Tree) AddString(text string) {
@@ -428,6 +435,14 @@
    t.rules[rule.String()] = rule
   }
  }
+ for name, r := range t.rules {
+  if r.name == "" {
+   r := &rule{name: name, id: t.ruleId}
+   t.ruleId++
+   t.rules[name] = r
+   t.PushBack(r)
+  }
+ }
 
  join([]func() {
   func() {
@@ -833,7 +848,7 @@
   case TypeRule:
    print("%v <- ", node)
    expression := node.( Rule ).GetExpression()
-   if expression != nil {printRule(expression)}
+   if expression != nilNode {printRule(expression)}
   case TypeDot:       print(".")
   case TypeName:      print("%v", node)
   case TypeCharacter,
@@ -1042,6 +1057,7 @@
    printLabel(out)
    printRestore(out)
    printEnd()
+  case TypeNil:
   default:
    fmt.Fprintf(os.Stderr, "illegal node type: %v\n", node.GetType())
   }
@@ -1053,7 +1069,7 @@
   if node.GetType() != TypeRule {continue}
   rule := node.( *rule )
   expression := rule.GetExpression()
-  if expression == nil {
+  if expression == nilNode {
    fmt.Fprintf(os.Stderr, "rule '%v' used but not defined\n", rule)
    print("\n  nil,")
    continue