bindingSet = new BindingSet(new Binding(varName, existingVariable), bindingSet);
bindingSet = new BindingSet(new Binding(varName, variable), bindingSet);
	protected static BindingSet addOrCheckDefinition(String varName, Variable variable, BindingSet bindingSet) {		Variable existingVariable = lookup(varName, bindingSet);		if (existingVariable == null) {			bindingSet = new BindingSet(new Binding(varName, existingVariable), bindingSet);		} else {			if (!existingVariable.equals(variable))				return null;		}		return bindingSet;	}