doc-src/Tutorial/Recdef/last

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

consts last :: 'a list => 'a
recdef last "measure (%xs. length xs)"
  "last [x]      = x"
  "last (x#y#zs) = last (y#zs)"