- Moved abs_def to drule.ML
- elim_defs now takes a boolean argument which controls the automatic
expansion of theorems mentioning constants whose definitions are
eliminated
types 'v binop = 'v => 'v => 'v
datatype ('a,'v) expr = Cex 'v
| Vex 'a
| Bex ('v binop) (('a,'v) expr) (('a,'v) expr)