type var = int

datatype dt = F | T | D of var * dt * dt

datatype bt =
    FF                          (*  0            *)
  | TT                          (*  1            *)
  | V    of var                 (*  Variable     *)
  | NEG  of bt                  (*  Negation     *)
  | AND  of bt * bt             (*  Conjunction  *)
  | OR   of bt * bt             (*  Disjunction  *)