type 'a sequence =
     (* constant sequence: Cs h ~= [h; h; h; ...] *)
 | Cs of 'a
     (* generic sequence: S (x, xs) ~= x::xs *)
 | S of 'a * 'a sequence lazy_t
     (* differential sequence: for ds ~= [d0; d1; d2; ...], A(x0, ds) ~= [x0;
        x0+d0; x0+d0+d1; x0+d0+d1+d2; ...] *)
 | A of 'a * 'a sequence lazy_t
     (* generalized geometric progression: for ds ~= [d0; d1; d2; ...],
        G(x0,ds) ~= [x0; x0*d0; x0*d0*d1; x0*d0*d1*d2; ...] *)
 | G of 'a * 'a sequence lazy_t
;;