"bpfilter.hh" has been used by Jan Reinkensmeier to specifiy a very specific kind of thermodynamic matcher. The matcher should only consider sub-structures which are composed of at least X base-pairs. We can implement this requirement as a semantic filter, which assesses a candidate after evaluation, here evaluation via algebra Algebras/alg_basepairMax.gap.
Since the semantic filter has to operate on the result type, and here it is defined as the product of three algebras, you might have to adapt the compontent (*.first.second) if you want to use this filter in your own code.