Provides a .by static constexpr function object.
When creating a binary function object of type Algorithm whose signature is Object x Predicate -> Return, nested_by<Algorithm> can be used as a base class to Algorithm. Doing so will provide a static constexpr member called by, which has the two following signatures:
In other words, nested_by is a curryed and flipped version of Algorithm. Note that the function object Algorithm must be default-constructible, since the algorithm will be called as Algorithm{}(arguments...).