doc-src/Tutorial/Recdef/gcd

added inform_file_processed, inform_file_retracted;
proper outer syntax setup;
more robust theory loader actions;

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))"