<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def make_subquery_heading(self):
 """ Create a new heading with removed attribute sql_expressions. Used by subqueries, which resolve the sql_expressions. """

        return Heading(dict(v.todict(), sql_expression=None) for v in self.attributes.values())