def __sub__(self, other):
        """
        Implementation of - operator, returns :class:`And` with error stop
        """
        if isinstance( other, basestring ):
            other = ParserElement._literalStringClass( other )
        if not isinstance( other, ParserElement ):
            warnings.warn("Cannot combine element of type %s with ParserElement" % type(other),
                    SyntaxWarning, stacklevel=2)
            return None
        return self + And._ErrorStop() + other

        Implementation of - operator, returns :class:`And` with error stop
        
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_vendor/pyparsing.py