docs: function calculate() {
    /*jshint validthis:true */
    var self = (this instanceof QueryBuilder) ? this : new QueryBuilder();
  
    var args = mlutil.asArray.apply(null, arguments);
  
    // TODO: distinguish facets and values
    var calculateClause = {
        constraint: args
    };
  
    self.calculateClause = calculateClause;
  
    return self;
  }