Here we introduce the first explicitly-coded actions - the previous
stages' movement between locations and ability to pick up and drop
items are ``intrinsics'' provided by the interpreter.

The new action provides the first puzzle: the player needs to unlock
the cell door entering the cell to obtain the coin.  The key is
necessary in order to open the door.

This stage uses the directives from the previous stage, plus
C<%nowhere>,
C<%at>,
C<%action>
and
C<%result>.