toString() {
    var pred = this.predicate ? this.predicate + ' ' : ''
      , type = this.type || ''
      , len = this.nodes.length
      , str = pred + type;
    if (len) {
      str += (type && ' and ') + this.nodes.map(function (expr) {
        return expr.toString();
      }).join(' and ');
    }
    return str;
  }
Return "<a> and <b> and <c>"

@return {String}
@api public