The reason is that you are using ___ instead of _. ___ is a pattern-matcher which can match zero or more sequences of arguments. In this example, it parses as follows: f = Null, g = Sequence[x^2 + y, x^3], x = x, y = y.

This can be prevented if you use _ instead of ___ throughout.

The reason is that you are using ___ instead of _. ___ is a pattern-matcher which can match zero or more sequences of arguments. In this example, it parses as follows: f = Null, g = Sequence[x^2 + y, x^3], x = x, y = y.

This can be prevented if you use _ instead of ___ throughout.