Defined in adapters.hpp.
This type should be a stateless trivially default constructible with an operator of signature void operator()(Point&, Element const&), which should modify the first argument in-place to contain the rho value of the second argument. The kernel of the rho function should be Green's \(\mathscr{R}\)-relation on the semigroup in question.
| Element | the type of elements. |
| Point | the type of the rho points. |
The third template parameter exists for SFINAE.