let one_of_three c =
let a = Array.create (3*(Array.length c)) None in
for i = 0 to (Array.length c)-1 do
match c.(i) with
| None -> ()
| Some (x,y,z) ->
a.(3*i+0) <- Some x ;
a.(3*i+1) <- Some y ;
a.(3*i+2) <- Some z
done ;
a