Edge magnet codes
pentagon edge ++++
hexagon, faces pentagon −−−−
hexagon, faces hexagon ++−−
north pole
south pole
Only P↔A and B↔B attract. P↔P and A↔A repel, so the coding alone forbids
every joint the target shape does not contain.
rejecting: edges reversed
coasting: magnets off
A tile that stays held with every latch open is stuck, not joined. It reverses
its free edges to shove off, then coasts on no magnets and comes back at a
different orientation.