docs: function (operator, field) {
              if (!this.query.where) {
                  return null;
              }
  
              function findExpression(expressions) {
                  var expression,
                      ret = null;
                  for (var i = 0; i < expressions.length; i++) {
                      expression = expressions[i];
                      if (expression instanceof Where) {
                          ret = findExpression(expression.expressions);
                      } else if (expression instanceof Comparator) {
                          if (expression.operator == operator && expression.field == field) {
                              ret = expression;
                          }
                      }
                      if (ret) {
                          return ret;
                      }
  
                  }
                  return ret;
              }
  
              return findExpression(this.query.where.expressions);
          }