def METHOD_NAME(self, s):
    parts = self._words.split(s)
    q = _Quantity(parts[0].rstrip('*/') or 1)
    for i in range(1, len(parts), 2):
        s = int(parts[i+1].rstrip('*/') or 1)
        if parts[i-1].endswith('/'):
            s = -s
        name = parts[i]
        if name not in self.quantities:
            if name[0] not in self._prefix or name[1:] not in self.quantities:
                raise ValueError('unknown unit: {}'.format(name))
            q *= _Quantity(self._prefix[name[0]]**s)
            name = name[1:]
        q *= self.quantities[name]**s
    return q