aspect CallerCrossRefs {
    // Fill in cross-references for callee -> caller
	boolean Program.callerCrossRefsFilledIn = false;
	private Set MethodDecl.privateCallers = new HashSet();
	
	public Set MethodDecl.callers() {
		Program root = getProgram();
		if (!root.callerCrossRefsFilledIn) {
		   root.fillInCallerCrossRefs();
		   root.callerCrossRefsFilledIn = true;
		}
		return privateCallers;
	}
	
	void ASTNode.fillInCallerCrossRefs() {
		for(int i=0; i<getNumChild(); i++) {
			getChild(i).fillInCallerCrossRefs();
		}
	}
	
	void MethodDecl.fillInCallerCrossRefs() {
		for (Iterator iter = calls().iterator(); iter.hasNext();) {
			MethodDecl m = (MethodDecl) iter.next();
			m.privateCallers.add(this);
		}
		super.fillInCallerCrossRefs();
	}

}