sub once {
  my ($self, $name, $cb) = @_;

  weaken $self;
  my $wrapper;
  $wrapper = sub {
# a special kind of circular reference
# CLOSURE, $wrapper keeps $self alive after return $wrapper (REFCNT+1)
# $wrapper refers to itself, CIRCULAR REFERENCE
    $self->unsubscribe($name => $wrapper);
    $cb->(@_);
  };
  $self->on($name => $wrapper); # potential CIRCULAR REFERENCE (on probably also stores $wrapper, but it probably also uses weaken )
  weaken $wrapper;

  return $wrapper;
}