code: function join (table, args, type) {
      this.typecheck && t.typecheck('join', [table, args, type]);
      if ('string'===typeof args) args = [args];
      var result = ['join',table,'on',args.join(' and ')];
      if (type) result.splice(0,0,type);
      return result.join(' ');
  }
docs: left join table on pk1=fk1 and pk2=fk2 ...