'Define action to perform if parsing fails at this expression.
Fail acton fn is a callable function that takes the arguments
C{fn(s,loc,expr,err)} where:
- s = string being parsed
- loc = location where expression match was attempted and failed
- expr = the parse expression that failed
- err = the exception thrown
The function returns no value.  It may throw C{L{ParseFatalException}}
if it is desired to stop parsing immediately.'

def setFailAction(self, fn):

    self.failAction = fn
    return self
