doc-src/Tutorial/Recdef/gcd

expand_proof now also takes an optional term describing the proposition
of the theorem to be expanded (to avoid problems with different theorems
having the same names).

recdef gcd "measure (%(m,n).n)"
  simpset "simpset() addsimps [mod_less_divisor]"
  "gcd (m, n) = (if n=0 then m else gcd(n, m mod n))"