def common(self, other):
		'''
			Find the shared part of two multipliers. This is the largest multiplier
			which can be safely subtracted from both the originals. This may
			return the "zero" multiplier.
		'''
		mandatory = min(self.mandatory, other.mandatory)
		optional = min(self.optional, other.optional)
		return multiplier(mandatory, mandatory + optional)
Find the shared part of two multipliers. This is the largest multiplier
			which can be safely subtracted from both the originals. This may
			return the "zero" multiplier.