SWI-Prolog -- numbervars/4

Availability:built-in

As numbervars/3, providing the following options:

functor_name(+Atom): Name of the functor to use instead of $VAR.
attvar(+Action): What to do if an attributed variable is encountered. Options are skip, which causes numbervars/3 to ignore the attributed variable, bind which causes it to treat it as a normal variable and assign the next '$VAR'(N) term to it, or (default) error which raises a type_error exception.^{117This
behaviour was decided after a long discussion between David Reitter,
Richard O'Keefe, Bart Demoen and Tom Schrijvers.}
singletons(+Bool): If true (default false), numbervars/4 does singleton detection. Singleton variables are unified with '$VAR'('_'), causing them to be printed as _ by write_term/2 using the numbervars option. This option is exploited by portray_clause/2 and write_canonical/2.^{bugCurrently
this option is ignored for cyclic terms.}