UP offers support in delivering meaningful
user feedback. Consider that the specified parser is to be applied in a
fancy IDE which registers parsing errors and makes propositions, on how
to fix these errors. In these cases, an implementation for the action code
of an error symbol could consist in telling the IDE about the exact
location of the unmatched input, as well as the list of viable symbols, with
which the parser can advance a step in the current state, reaching an
accepting state. These symbols are 