theorem database now also indexes constants "Trueprop", "all",
"==>", "=="; thus thms_containing, findI etc. may retrieve more rules;
Sep2 = Main +
consts sep :: 'a list => 'a => 'a list
recdef sep "measure length"
"sep (x#y#zs) = (%a. x # a # sep zs a)"
"sep xs = (%a. xs)"
end