I have updated pack pac to v1.6.8. If the log below is reproduced on your terminal, then perhaps you can play around some zdd programming, referring to a block of comment lines in library(‘pac/zdd/zdd.pl’) as cheat sheet.
% swipl
?- pack_install(pac).
% Contacting server at https://www.swi-prolog.org/pack/query ... ok
% Pack `pac' is already installed @1.6.8
?- use_module(library(pac)).
?- use_module(zdd(zdd)).
zdd: ?- use_state(S), X<<pow([a,b]), cofact(X, t(A, L, R), S), psa(X, S), psa(L, S), psa(R, S).
zid = 3
[]
[b]
[a]
[a,b]
-------------------
zid = 2
[]
[b]
-------------------
zid = 2
[]
[b]
-------------------
S = ..,
X = 3,
A = a,
L = R, R = 2.