求"xy"IndexOf("x")>0的表达式树，即要Expression.Lambda>(exp,pa[])的exp,该如何处理

ParameterExpression numParam = Expression.Parameter(typeof(string), "str");
        ConstantExpression b = Expression.Constant(true, typeof(bool));
        MethodCallExpression methodCall = Expression.Call(typeof(string).GetMethod("IsNullOrEmpty"), numParam);
        BinaryExpression isEqual = Expression.NotEqual(methodCall, b);
        Expression<Func<string, bool>> lambda1 =
            Expression.Lambda<Func<string, bool>>(
                isEqual,
                new ParameterExpression[] { numParam });