Instantiation confusion

[flexoron]

?- use_module(library(freeze)).
   true.
?- [user].
:- module(b, [b/0]).
b:b.

?- freeze(X,(Y=Z)), freeze(Y,b:P), bagof(Y=Y,X=Y,[Z=Z]), P=b, X=true. % notice P=b
   error(instantiation_error,call/1).

?- freeze(X,(Y=Z,P=b)), freeze(Y,b:P), bagof(Y=Y,X=Y,[Z=Z]), X=true. % now P=b 'freeze-shifted'
   error(instantiation_error,call/1).

?- freeze(X,(Y=Z,P=b)), freeze(Y,b:P), bagof(Y=Y,X=Y,[Z=Z]), P=b, X=true. % both in place and it works, why?
   X = true, Y = true, Z = true, P = b.
?- 

A problem I see is that b:b is called twice:

?- [user].
:- module(b, [b/0]).
b:b :- write(trace),nl.

?- freeze(X,(Y=Z,P=b)), freeze(Y,b:P), bagof(Y=Y,X=Y,[Z=Z]), P=b, X=true.
trace
trace
   X = true, Y = true, Z = true, P = b.

% Four are better than two
?- freeze(X,(Y=Z,P=b)), freeze(Y,(P:P,P:P)), bagof(Y=Y,X=Y,[Z=Z]), P=b, X=true.
trace
trace
trace
trace
   X = true, Y = true, Z = true, P = b.
?-

[UWN]

Shorter:

?- freeze(X,a),freeze(Y,b),X=Y.
   X = Y, freeze:freeze(X,(a,b)) % fine, one possible answer
|  X = Y, freeze:freeze(X,a), freeze:freeze(X, b). % another possible answer
?- freeze(X,a),freeze(Y,b), bagof(t,X=Y,_).
   X = Y, freeze:freeze(X,((a,a,b),b)), unexpected.
?- freeze(X,a),freeze(Y,b), setof(t,X=Y,_).
   X = Y, freeze:freeze(X,((a,a,b),b)), unexpected.