Library Coq.PArith.POrderedType
Note that the last module fulfills by subtyping many other
    interfaces, such as DecidableType or EqualityType. 
 
OrderedType structure for positive numbers
Note that Positive_as_OT can also be seen as a UsualOrderedType
   and a OrderedType (and also as a DecidableType). 
 
An order tactic for positive numbers
Note that p_order is domain-agnostic: it will not prove
    1<=2 or x<=x+x, but rather things like x<=y -> y<=x -> x=y. 
