The parser supports automatic error correction of its input. The strategy used involves attempting to either push a new token onto the input, replacing an erroneous symbol, or discarding an erroneous symbol. Such action is only taken if it is guaranteed that the parser can continue parsing and read at least one more symbol from its input.
If the correction strategy fails, then error recovery is invoked.
The parser allows the inclusion of grammar productions of the form:
non-terminal -> :error
This means that the parser accepts an erroneous string of tokens as constituting an occurrence of the non-terminal. Such productions may be used to skip over portions of input when attempting to recover from an error. The action associated with such an error is specified by a form in the same way as for ordinary actions. The action may perform manipulation of the parser state and input.
LispWorks User Guide and Reference Manual - 21 Dec 2011