theorem database now also indexes constants "Trueprop", "all",
"==>", "=="; thus thms_containing, findI etc. may retrieve more rules;
Last = List +
consts last :: 'a list => 'a
recdef last "measure (%xs. length xs)"
"last [x] = x"
"last (x#y#zs) = last (y#zs)"
end