Chocolate in PB

[debug]  ?- plvar(X),X=1,Y=X,rtrace(Y==X).
^  Call: (67) [system] must_atomic(system:metaterm_flags(global,set,metaterm_disabled))
^  Unify: (67) [system] must_atomic(system:metaterm_flags(global,set,metaterm_disabled))
^  Exit: (67) [system] must_atomic(system:metaterm_flags(global,set,metaterm_disabled))
   Call: (67) [system] catch((call((trace,system:wo_metavmi('$attvar':wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true))),notrace,deterministic(_G22992),true))*-
> (_G22992==true->must_atomic(system:metaterm_flags(global,~,metaterm_disabled));must_atomic(system:metaterm_flags(global,~,metaterm_disabled));must_atomic(system:metaterm_flags(gl
obal,set,metaterm_disabled)),fail);must_atomic(system:metaterm_flags(global,~,metaterm_disabled)),fail),_G22969,(must_atomic(system:metaterm_flags(global,~,metaterm_disabled)),thro
w(_G22969)))
   Unify: (67) [system] catch(system: (call((trace,system:wo_metavmi('$attvar':wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true))),notrace,deterministic(_G22992)
,true))*-> (_G22992==true->must_atomic(system:metaterm_flags(global,~,metaterm_disabled));must_atomic(system:metaterm_flags(global,~,metaterm_disabled));must_atomic(system:metaterm
_flags(global,set,metaterm_disabled)),fail);must_atomic(system:metaterm_flags(global,~,metaterm_disabled)),fail),_G22969,system: (must_atomic(system:metaterm_flags(global,~,metater
m_disabled)),throw(_G22969)))
   Call: (70) [system] wo_metavmi('$attvar':wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true)))
   Unify: (70) [system] wo_metavmi('$attvar':wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true)))
^  Call: (75) [system] must_atomic(system:ignore(set_no_metavmi(_G23048,0+1)))
^  Unify: (75) [system] must_atomic(system:ignore(set_no_metavmi(_G23048,0+1)))
^  Exit: (75) [system] must_atomic(system:ignore(set_no_metavmi(0,0+1)))
   Call: (75) [system] catch((call((trace,'$attvar':wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true)),notrace,deterministic(_G23119),true))*-> (_G23119==true->m
ust_atomic(system:set_no_metavmi(_G23054,0));must_atomic(system:set_no_metavmi(_G23054,0));must_atomic(system:ignore(set_no_metavmi(0,0+1))),fail);must_atomic(system:set_no_metavmi
(_G23054,0)),fail),_G23096,(must_atomic(system:set_no_metavmi(_G23054,0)),throw(_G23096)))
   Unify: (75) [system] catch(system: (call((trace,'$attvar':wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true)),notrace,deterministic(_G23119),true))*-> (_G23119
==true->must_atomic(system:set_no_metavmi(_G23054,0));must_atomic(system:set_no_metavmi(_G23054,0));must_atomic(system:ignore(set_no_metavmi(0,0+1))),fail);must_atomic(system:set_n
o_metavmi(_G23054,0)),fail),_G23096,system: (must_atomic(system:set_no_metavmi(_G23054,0)),throw(_G23096)))

Chocolate in PB

   Call: (78) [$attvar] wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true))
   Unify: (78) [system] wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true))


^  Call: (83) [system] must_atomic(system:ignore(set_no_metavmi(_G23178,1+1)))
^  Unify: (83) [system] must_atomic(system:ignore(set_no_metavmi(_G23178,1+1)))
^  Exit: (83) [system] must_atomic(system:ignore(set_no_metavmi(1,1+1)))
   Call: (83) [system] catch((call((trace,system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true),notrace,deterministic(_G23249),true))*-> (_G23249==true->must_atomic(system:set_
no_metavmi(_G23184,1));must_atomic(system:set_no_metavmi(_G23184,1));must_atomic(system:ignore(set_no_metavmi(1,1+1))),fail);must_atomic(system:set_no_metavmi(_G23184,1)),fail),_G2
3226,(must_atomic(system:set_no_metavmi(_G23184,1)),throw(_G23226)))
   Unify: (83) [system] catch(system: (call((trace,system: ('$metaterm_call'(==,2,_G19696{..},_G22902),true),notrace,deterministic(_G23249),true))*-> (_G23249==true->must_atomic(sy
stem:set_no_metavmi(_G23184,1));must_atomic(system:set_no_metavmi(_G23184,1));must_atomic(system:ignore(set_no_metavmi(1,1+1))),fail);must_atomic(system:set_no_metavmi(_G23184,1)),
fail),_G23226,system: (must_atomic(system:set_no_metavmi(_G23184,1)),throw(_G23226)))
   Exit: (78) [system] wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},1),true))
   Exit: (70) [system] wo_metavmi('$attvar':wo_metavmi(system: ('$metaterm_call'(==,2,_G19696{..},1),true)))
Y = 1,