doc-src/Tutorial/Recdef/gcd

Added Tanja's Detects and Reachability theories. Also
changed object-quantifiers to meta-quantifiers in ball_constrains_UN/INT...

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))"