match op1 with
[ None ⇒ match op2 with [ None ⇒ true | Some _ ⇒ false ]
| Some x1 ⇒ match op2 with [ None ⇒ false | Some x2 ⇒ f x1 x2 ]
match op1 with
[ None ⇒ match op2 with [ None ⇒ true | Some _ ⇒ false ]
| Some x1 ⇒ match op2 with [ None ⇒ false | Some x2 ⇒ f x1 x2 ]