def set(self, names):
names = tuple(names)<EOL>if any(not self.has_variable(name) for name in names):<EOL><INDENT>raise ValueError('<STR_LIT>'.format(<EOL>set(names) - set(self._variables)))<EOL><DEDENT>return Expression({VariableSet(names): <NUM_LIT:1>})<EOL>
Return variables as a set expression.

        This returns an :class:`.Expression` containing a
        :class:`.VariableSet`.