don't print env[0]'s bindings in stack trace

These made the stack trace unreadble and were not particularly
helpful.

eval/eval.go

@@ -39,15 +39,19 @@ func EvalLevel() int {
 func printEnv(env *lob.Environment) string {
 	buf := bytes.NewBufferString(env.ValueString(nil))
 	buf.WriteRune('{')
-	first := true
-	for key, value := range env.Varmap() {
-		if !first {
-			buf.WriteString("; ")
+	if env.Level() == 0 {
+		buf.WriteString("...")
+	} else {
+		first := true
+		for key, value := range env.Varmap() {
+			if !first {
+				buf.WriteString("; ")
+			}
+			buf.WriteString(key.ValueString(nil))
+			buf.WriteRune('=')
+			buf.WriteString(value.ValueString(nil))
+			first = false
 		}
-		buf.WriteString(key.ValueString(nil))
-		buf.WriteRune('=')
-		buf.WriteString(value.ValueString(nil))
-		first = false
 	}
 	buf.WriteRune('}')
 	return buf.String()
@@ -58,7 +62,7 @@ func printEnv(env *lob.Environment) string {
 func PrintStack() {
 	for i := 0; i < len(evalStack); i += 2 {
 		// take care to indent the environment line as appropriate
-		levelStr := fmt.Sprintf("#%d ", i/2)
+		levelStr := fmt.Sprintf(":%d ", i/2)
 		fmt.Printf("%s%s\n%*s%s\n", levelStr, evalStack[i],
 			len(levelStr), "",
 			printEnv(evalStack[i+1].(*lob.Environment)))

lob/environment.go

@@ -101,6 +101,10 @@ func (env *Environment) Varmap() VarMap {
 	return env.val
 }
 
+func (env *Environment) Level() int {
+	return env.level
+}
+
 func (env *Environment) Vars(noparents bool) Object {
 	vars := env.allvarmap(noparents)
 	var result Object = Nil