'Check whether a callable needs to be processed.  The ``spec``
refers to a unique identifier for the callable.
Return True if processing is needed and False otherwise. If
the callable needs to be processed, it will be marked as
processed, assuming that the caller will procces the callable if
it needs to be processed.'

def processSpec(self, spec):

    if (spec in self._seen_files):
        return False
    self._seen_files.add(spec)
    return True
