doc-src/Tutorial/Recdef/gcd

reverted to old proof of dominoes_tile_row, given new treatment of #2+...

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))"