It should be treat instead of thread in attvar description.
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skip, which causes numbervars/3 to ignore the attributed variable,
bindwhich causes it to treat it as a normal variable and assign the next
'$VAR'(N) term to it, or (default)
errorwhich raises a
type_errorexception.109This behaviour was decided after a long discussion between David Reitter, Richard O'Keefe, Bart Demoen and Tom Schrijvers.
false), numbervars/4 does singleton detection. Singleton variables are unified with
'$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.