I wrote this earlier today while wrangling with the formation of the opening section of Si el clima:
The idea of ‘invertible’ parametric counterpoint would be to have an exchangeability of properties between lines so that the material of say the top line can – by way of operations on a variety of parameters – become the material of the bottom line, and vice versa. This could apply simply to one parameter, or to many. This gives global shape to sections, rather than simply to lines, and functions somewhat as the ‘subject’ of the given section. Nonetheless, an unmediated and obvious presentation of such an inversion would likely come across as a bit naff and smell too much of thematism. In this sense it is necessary to confront this kind of approach with other processes, foremost of which would be independent processes in each individual lines, meaning that by the time the counterpoint has inverted (if one were to invert entirely), the materials of each line would have themselves changed into something else.