This is called a default rule. If there are no rules to match the inputq and there is no default rule, the translation stops with a message. Actions Possible action s include rule set selection, inputq editing and parsing. Action s may also make use of the underlying virtual machine which consists of 2 stacks, memory, an ALU and control flow. The new phrase is the set of strings which gets stuff ed back into the inputq when the match phrase is removed.
Some folks prefer to read a manual while others prefer to learn by example.
Some of these applications are very powerful yet they all remain simple. A PostScript version of the original paper is also available.