+ // Used to iterate over symbols which are referenced in expressions
+ // but not defined.
+ typedef Unordered_set<std::string>::const_iterator referenced_const_iterator;
+
+ referenced_const_iterator
+ referenced_begin() const
+ { return this->symbol_references_.begin(); }
+
+ referenced_const_iterator
+ referenced_end() const
+ { return this->symbol_references_.end(); }
+
+ // Return whether a symbol is referenced but not defined.
+ bool
+ is_referenced(const std::string& name) const
+ {
+ return (this->symbol_references_.find(name)
+ != this->symbol_references_.end());
+ }
+
+ // Return whether there are any symbols which were referenced but
+ // not defined.
+ bool
+ any_unreferenced() const
+ { return !this->symbol_references_.empty(); }
+