sub decodable
{
    # should match the Content-Encoding values that decoded_content can deal with
    my $self = shift;
    my @enc;
    # XXX preferably we should determine if the modules are available without loading
    # them here
    eval {
        require IO::Uncompress::Gunzip;
        push(@enc, "gzip", "x-gzip");
    };
    eval {
        require IO::Uncompress::Inflate;
        require IO::Uncompress::RawInflate;
        push(@enc, "deflate");
    };
    eval {
        require IO::Uncompress::Bunzip2;
        push(@enc, "x-bzip2");
    };
    # we don't care about announcing the 'identity', 'base64' and
    # 'quoted-printable' stuff
    return wantarray ? @enc : join(", ", @enc);
}