sub tail {
        my $list = shift;
        if ( ref( $list->[1] eq 'CODE' ) { return $s->[1]->() );
        else { return $s->[1] }
}


##</code><code>##


sub upto {
        my ($m, $n) = @_;
        return if $m > $n;
        node( $m, promise { upto($m+1, $n) } );
}


##</code><code>##


sub promise (&) { $_[0] }


##</code><code>##


sub node {
        my ($head, $tail) = @_;
        return [$head, $tail];
}


##</code><code>##


'Can't call method "promise" on an undefined value.'


##</code><code>##


Odd number of elements in anonymous hash at stream.pl line 19.
Use of uninitialized value in anonymous hash ({}) at stream.pl line 19.


##</code><code>##


        node( $m, promise ( upto($m+1, $n) ) );