code: function () {
                  var negate, p = input.charAt(i + 1);
  
                  if (input.charAt(i) === '-' && (p === '@' || p === '(')) { negate = $('-') }
                  var o = $(this.sub) || $(this.entities.dimension) ||
                          $(this.entities.color) || $(this.entities.variable) ||
                          $(this.entities.call);
  
                  if (negate) {
                      o.parensInOp = true;
                      o = new(tree.Negative)(o);
                  }
  
                  return o;
              }
docs: An operand is anything that can be part of an operation, such as a Color, or a Variable