refers to provable and the backslash (
is normally used to indicate negation in Prolog). In contrast to the ISO
standard, but compatible with several other Prolog systems, SWI-Prolog
\+/1 as a control
structure. This implies that its argument is compiled as part of
the enclosing clause and possible variables in goal positions are
translated to call/1.
As a result, if such a variable is at runtime bound to a (!/0),
the cut is scoped to the call/1
call rather than the enclosing \+/1.