code: function (op, other) {
          return new(tree.Dimension)
                    (tree.operate(op, this.value, other.value),
                    this.unit || other.unit);
      }
docs: In an operation between two Dimensions, we default to the first Dimension's unit, so `1px + 2em` will yield `3px`. In the future, we could implement some unit conversions such that `100cm + 10mm` would yield `101cm`.