Reductionops.Stack.map & Reduction.iterate_whd_gen
cbn: args list instead of arg number
Cst_stack before stack (abstraction leak in whd_gen)
Reduction.Stack.Fix/Case stores Cst_stack.t
Cbn reduces Pos.compare p~1 q~1 to Pos.compare p q

(refolding of cbn is smarter)