adds an assignment clause to this statement
        :param clause: the clause to add
        :type clause: AssignmentClause

def add_assignment_clause(self, clause):
        """
        adds an assignment clause to this statement
        :param clause: the clause to add
        :type clause: AssignmentClause
        """
        if not isinstance(clause, AssignmentClause):
            raise StatementException("only instances of AssignmentClause can be added to statements")
        clause.set_context_id(self.context_counter)
        self.context_counter += clause.get_context_size()
        self.assignments.append(clause)