So, isn't "isa" and the "£" merely things which can be added by
programmers by changing the grammar? That was one of the design goals
of the language.
With a changeable grammar this applies to everything except the
changeability itself. But the type system is part of the language
core. As such 'isa' and 'like' or assignment and binding semantics
need a definition.
"The unavoidable price of reliability is simplicity" -- C.A.R. Hoare
1 + 2 + 3 + 4 + ... = -1/12 -- Srinivasa Ramanujan