Previous version did (ACTION 'UPDATE (CDR REST) ACTION) when a new
action had to be made. This caused default actions to propagate
through the trie.
While this isn't bad (it causes #TRU to be equal to #TRUE when that
was the last prefix in the tree), it can cause unexpected errors at
runtime, and no program should ever depend on its behavior.
The current solution is to make a new PASS readtable with a default
error action.
Peter McGoron