start using return stack to track evaluation location

words.go

@@ -9,11 +9,11 @@ type Dictionary map[string]Word
 type Word struct {
 	Name   string
 	Impl   func() error
-	Source string
+	Source []string
 }
 
 // AddWord inserts a new word into the dictonary, optionally overwriting the existing word
-func (d Dictionary) AddWord(name string, impl func() error, source string) {
+func (d Dictionary) AddWord(name string, impl func() error, source []string) {
 	d[name] = Word{
 		Name:   name,
 		Impl:   impl,