def dock(self, other):
		'''
			"Dock" another mult from this one (i.e. remove part of the tail) and
			return the result. The reverse of concatenation. This is a lot trickier.
			e.g. a{4,5} - a{3} = a{1,2}
		'''
		if other.multiplicand != self.multiplicand:
			raise Exception("Can't subtract " + repr(other) + " from " + repr(self))
		return mult(self.multiplicand, self.multiplier - other.multiplier)