Add Ƙ (where) quick

[lynn]

Let F be a monadic link. FƘ is a dyad that maps (x, y) to [t for t in x if F(t) == y].

So, …LƘ4 means “filter to elements whose Length is 4”.

This is a shorter way to write …L=¥Ƈ4 or …L=4ƊƇ.