adds a where clause to this statement
        :param clause: the clause to add
        :type clause: WhereClause

def add_where_clause(self, clause):
        """
        adds a where clause to this statement
        :param clause: the clause to add
        :type clause: WhereClause
        """
        if not isinstance(clause, WhereClause):
            raise StatementException("only instances of WhereClause can be added to statements")
        clause.set_context_id(self.context_counter)
        self.context_counter += clause.get_context_size()
        self.where_clauses.append(clause)