def __sub__(self, other):
        """
        Implementation of - operator, returns C{L{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 C{L{And}} with error stop
        
src/pkg_resources/_vendor/pyparsing.py