Strip a prefix from the keys of another dictionary, returning a Bunch containing only valid key, value pairs.

def partial(cls, prefix, source):
        """Strip a prefix from the keys of another dictionary, returning a Bunch containing only valid key, value pairs."""
        match = prefix + "."
        
        matches = cls([(key[len(match):], source[key]) for key in source if key.startswith(match)])
        
        if not matches:
            raise ValueError()
        
        return matches