Twinning a) white to move, b) black to move. Find the a) and b) "same
nature" legal last move.
The move's nature is determined by the type of the piece/pieces involved
(K, Q, R, B, N, P) and the type of movement (simple, straight promotion,
simple capture, capturing promotion, e.p., pawn double step, O-O, O-O-O).
An ELM problem is not (in general) a Last Move type D one. The diagram
could present many legal candidates to be the last move, but there is
just one for a) and just one for b) that are of the same nature. On
the contrary, all the Last Move type DD problems are particular ELM
Taken from a mail by Roberto Roasorio on the Retro Mailing List from
June 10th, 2007