These tests can be used both inside and outside the condition evaluation process, and also can be used in queries about past break levels.
break_level(
N)
max_inv(
MaxInv)
private(
Priv)
goal_private/1
, this condition refers initially to
an unbound variable and can be used to store an arbitrary Prolog
term. However, it is strongly recommended that Priv is used as an
open ended list, see Storing User Information in the Backtrace.