Ian:

The Law has another beneficial effect which we don't mention in the paper:
It facilitates proofs by structural induction. This comes out
clearly in the object version of the Law, if we assume that we
don't have method arguments. Then the Law is identical in spirit
to the Compositionality Principle and the major motivation behind that one
is structural induction.

If you agree, please can find an appropriate place in the paper.
I think it is an important property of the Law.

-- Karl