@Override
    public List<AbstractExpression> bindingToIndexedExpression(AbstractExpression expr) {
        if (equals(expr)) {
            return s_reusableImmutableEmptyBinding;
        }
        return null;
    }
@ Override public List < AbstractExpression > bindingToIndexedExpression ( AbstractExpression expr ) { if ( equals ( expr ) ) { return s_reusableImmutableEmptyBinding ; } return null ; }
Otherwise, there is no binding possible, indicated by a null return.