doc-src/Tutorial/Recdef/gcd

theorem database now also indexes constants "Trueprop", "all",
"==>", "=="; thus thms_containing, findI etc. may retrieve more rules;

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))"