sub c2_out {
    my( $fh, $x, $y )= @_;
    if(  defined $y  ) {
        print $fh pack "C3",
            $x & 0xff, ($x>>8) | ($y&0xf)<<4, $y>>4;
    } else {
        print $fh pack "CC", $x & 0xff, $x>>8;
    }
}

sub c2_in {
    my( $fh )= @_;
    my $len= read( $fh, $c, 3 );
    return   if  ! $len;
    die "Extra trailing byte (",unpack("C",$c),")"
        if  1 == $len;
    my( $x, $y, $z )= unpack "C*", $c;
    $x |= ($y&0xf)<<8;
    return $x   if  ! defined $z;
    return( $x, $z<<4 | $y>>4 );
}