theorem database now also indexes constants "Trueprop", "all",
"==>", "=="; thus thms_containing, findI etc. may retrieve more rules;
types 'v binop = 'v => 'v => 'v
datatype ('a,'v) expr = Cex 'v
| Vex 'a
| Bex ('v binop) (('a,'v) expr) (('a,'v) expr)