  Let No be Conway's class of surreal numbers. I will make explicit the notion
of a function f on No recursively defined over some family of functions. Under
some "tameness" and uniformity condition, f must satisfy some interesting
properties; in particular, the supremum of the class of element greater or
equal to a fixed d in No is actually an element of No. For similar reasons, the
concatenation function x:y cannot be defined recursively in a uniform way over
polynomial functions.
