prolog_flag executable (obs from Aubrey Barnard).

docs/yap.tex

@@ -7595,6 +7595,11 @@ token is converted to a list of atoms, @code{chars}, to a list of integers,
 @code{codes}, or to a single atom, @code{atom}. If @var{Value} is bound, set to
 the corresponding behavior. The default value is @code{codes}.
 
+@item executable
+@findex executable(yap_flag/2 option)
+@* Read-only flag. It unifies with an atom that gives the
+original program path.
+
 @item  fast
 @findex fast (yap_flag/2 option)
 @*