type oexpr =
| OPlane of int*int
| OSphere of int*int*int;;

type instruction =
| Put of oexpr
| Assign of string*expr;;

Now I want to match on OPlane or OSphere:

let func instructions = match instructions with
| [] -> ()
| a::b -> match a with
       | Put exp -> match exp with
              | OPlane -> ()
              | OSphere -> ();;