The college professor who carried on a letter exchange with a kind pensioner who proved that the speed of light could be exceeded told me of another letter exchange, this time with another professional mathematician.
The letter came from England, and it accompanied some sort of document or artifact that the correspondent wanted the college professor to look over and return. The mathematician took the effort of including a stamped return envelope with the remark, "By the commutative law for postage, I have placed the same amount of postage on the return envelope as I have on the outgoing envelope."
The professor, however, could not use the return envelope the correspondent provided. He explained, "Unfortunately, the commutative law for postage does not apply to correspondence between the United States and the United Kingdom due to events of slightly more than 200 years ago."