our $__frag;  # points to fragment under active construction

sub doc(&) {
    my ($content_fn) = @_;
    local $__frag = [undef,undef,undef];
    $content_fn->();
    $__frag->[2][0];
}

sub _elem {
    my ($elem_name, $content_fn) = @_;
    # an element is represented by the triple [name, attrs, children]
    my $elem = [$elem_name, undef, undef];
    do { local $__frag = $elem; $content_fn->() };
    push @{$__frag->[2]}, $elem;
}

sub _attr {
    my ($attr_name, $val) = @_;
    push @{$__frag->[1]}, [$attr_name, $val];
}

sub text($) {
    push @{$__frag->[2]}, @_;
}