better error for transitive verbs w/o object
player no longer allowed to take (heavy) disks
supported 'put left disk on right rod' syntax
This led to the realization that it's probably a bad idea to have a
'middle disk', so renamed it to the 'medium disk'.
Also made inverse_lookup a globally defined sub, because it's so useful.
mentioned open/close in help
Too tired to code tonight; but enough energy to list what needs doing.