These tests can be used both inside and outside the condition evaluation process, and also can be used in queries about past break levels.
We are at (or focused on) break level N (N = 0 for the outermost break level).
The last invocation number used within the current break level is MaxInv. Note that this invocation number may not be present in the backtrace (because the corresponding call exited determinately).
The private information associated with the break level is 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 be used as an
open ended list, see Storing User Information in the Backtrace.