Splits the AST if its operation is `split_on` (i.e., return all the arguments). Otherwise, return a list with
        just the AST.

def split(self, split_on):
        """
        Splits the AST if its operation is `split_on` (i.e., return all the arguments). Otherwise, return a list with
        just the AST.
        """
        if self.op in split_on: return list(self.args)
        else: return [ self ]