// checks for undefined elements
// 
// Collaboration
// role System played by Grammar
// role UsedElementHolder played by Expression
// role Element played by NonTerminal

// System = List(Element) List(UsedElementHolder).
// UsedElementHolder = List(Element).
// Element = Ident.

Grammar{
{{
	// where we want to go
        String si = "from NonTerminal to edu.neu.ccs.demeter.Ident";
	String getDef = "from Grammar bypassing Expression to NonTerminal";
	String getUsed = "from Grammar through Expression to NonTerminal";

	int errors = 0;

	int isDefinedExactlyOnce(ClassGraph cg){
		checkDefined(cg, getDefinedThings(cg));
		return errors;
	}

	HashSet getDefinedThings(final ClassGraph cg){
		return (HashSet)cg.traverse(this, getDef, new Visitor(){
                  HashSet rv  = new HashSet();
		  void before(NonTerminal v){
		    Ident i = (Ident) cg.fetch(v,si);
		    if (rv.contains(i)) {
		      errors++;
		      System.out.println("The element "+ i
		      + " is defined more than once.");}
                    else rv.add(i);
		  }
	          public Object getReturnValue(){ return rv; } 
	        });		
	}

	void checkDefined(final ClassGraph cg, final HashSet definedSet){
		cg.traverse(this, getUsed, new Visitor(){
		  void before(NonTerminal v){
		    Ident i = (Ident) cg.fetch(v,si);
		    if(!definedSet.contains(i)) {
		        System.out.println("The element "+ i
		        + " is undefined.");
		        errors++; }
                  }
                });
	}
}}
}