sub doit {
    my( $self, $a, $b ) = @_;
    die unless $self =~ __PACKAGE__ and look_likenumber( $a ) and looks_like_number( $b );
    ## do some stuff before
    $self->{thing} += $a;
    $self->{thing} *= $b;
    ## do some stuff after
    return $self->{thing};
}