guest, I understand sub-typing is a popular feature in many 
other programming languages, but Clay doesn't have it. I 
personally find sub-typing to be an unsound feature in the 
presence of mutation. And, mutation is definitely a far more 
important feature than sub-typing. If you do have a very 
practical and concrete use case for sub-typing where Clay's 
other features don't suffice, then I will think about it.