code: def add_where_clause(self, clause):
          
          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)
docs: adds a where clause to this statement
          :param clause: the clause to add
          :type clause: WhereClause