doc-src/Tutorial/Recdef/gcd

- Datatype package now also supports arbitrarily branching datatypes
(using function types).
- Added new simplification procedure for proving distinctness of
constructors.
- dtK is now a reference.

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))"