doc-src/Tutorial/Recdef/gcd

added theorems subset_insertD, singleton_insert_inj_eq, subset_singletonD

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))"