theorem database now also indexes constants "Trueprop", "all",
"==>", "=="; thus thms_containing, findI etc. may retrieve more rules;
Itrev = Main +
consts itrev :: 'a list => 'a list => 'a list
primrec
"itrev [] ys = ys"
"itrev (x#xs) ys = itrev xs (x#ys)"
end