[jztgfqne] Pawn which can move backwards

Add to a chess pawn's movement the ability to move backward: one step backward into an empty square or a diagonally backwards capture.  Somewhat similar to the wazir-ferz hybrid, except it can't move sideways.

A possible interesting restriction: Consider the triangle formed by pawn and the two diagonals southwest and southeast emanating from a white pawn.  A pawn may only move backwards if there is an enemy piece somewhere in that triangular area, the pawn's cone of backward vision.  A pawn can "advance" toward the enemy.  A far advanced pawn has a larger backwards cone, so is more powerful because it can move backwards more frequently.