code: def prune(self, var, value, removals):
          "Rule out var=value."
          self.curr_domains[var].remove(value)
          if removals is not None: removals.append((var, value)f prune(self, var, value, removals):
          "Rule out var=value."
          self.curr_domains[var].remove(value)
          if removals is not None: removals.append((var, value))
docs: Rule out var=value.