Goal is unified with the conjunction of goals blocked on Var.
term, must be var
If some goal is blocked on the variable Var, or Var has
attributes that can be interpreted as a goal (see lib-atts),
then that goal is unified with Goal. If no goals are blocked,
Goal is unified with the atom
true. If more than one
goal is blocked, a conjunction is unified with Goal.
Var is not a variable