From a bug report, attempting to move
diagonally when poly'd into grid bug form doesn't give any useful
feedback in the general case, and in the specific case of using 'u' to
try to move northeast with vi-style keys, it performs #untrap instead.
Diagonal directions were being classified as non-movement when in grid
bug form, so the feedback was usually just "unknown command". But 'u'
is bound to untrap as a a convenience to players who use num_pad-style
movement keys. (Move commands don't actually reach the assigned key
part of command handling, so for forms other than grid bug, !num_pad 'u'
moves NE despite the untrap function given to that key.)
Split the diagonal handling out from movement command recognition.
It now gives "you can't get there from here..." if player tries to move
diagonally as a grid bug. For direction prompts, it now gives "you can't
orient yourself that direction" instead of popping up the command assist
display. (In the prompt string showing likely candidate directions for
digging, diagonal handling for grid bugs is academic because they aren't
strong enough to wield pick-axes.)
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