call_once(X)?
X= !?

X= (write(1),!,write(2))?

X=((A=1;A=2;A=3),write(A),fail)?
X=(A={1;2;3},write(A),fail)?
Y=(A={1;2;3},write(A),fail)?
X=Y?
.
call_once(succeed)?
call_once(true)?
call_once((true,true))?

