code: function union(callbacks, wrap) {
      if (arguments.length === 1 || arguments.length === 2 && typeof wrap === 'boolean') {
        if (!Array.isArray(callbacks)) {
          callbacks = [callbacks];
        }
        for (var i = 0, l = callbacks.length; i < l; i++) {
          this._statements.push({
            grouping: 'union',
            clause: 'union',
            value: callbacks[i],
            wrap: wrap || false
          });
        }
      } else {
        callbacks = normalizeArr.apply(null, arguments).slice(0, arguments.length - 1);
        wrap = arguments[arguments.length - 1];
        if (typeof wrap !== 'boolean') {
          callbacks.push(wrap);
          wrap = false;
        }
        this.union(callbacks, wrap);
      }
      return this;
    }
docs: Add a union statement to the query.