eval1 env (If0 c t e) =
  if eval1 env c == 0
  then eval1 env t
  else eval1 env e
eval1 env (If0 c t e) =
  if eval1 env c == 0
  then eval1 env t
  else eval1 env e
eval1 env (If0 c t e) =
  if eval1 env c == 0
  then eval1 env t
  else eval1 env e