{
    my %prev;
    sub c_out {
        my( $fh, $c )= @_;
        if(  ! defined $c  ) {
            print $fh pack "CC", $c & 0xff, $c >> 8
                if  exists $prev{$fh};
        } elsif(  exists $prev{$fh}  ) {
            print $fh pack "CC", $prev{$fh} | (($c&0xf)<<4), $c>>4;
        } else {
            print $fh pack "C", $c & 0xff;
            $prev{$fh}= ( $c>>8 ) & 0xf;
            return;
        }
        delete $prev{$fh};
    }
}
{
    my %prev;
    sub c_in {
        my( $fh )= @_;
        my $c;
        if(  exists $prev{$fh}  ) {
            if(  ! read( $fh, $c, 1 )  ) {
                die "Invalid trailing nybble ($prev{$fh})"
                    unless  0 == delete $prev{$fh};
                return;
            }
            return delete($prev{$fh})<<8 | unpack("C",$c);
        } else {
            my $len= read( $fh, $c, 2 );
            die "Extra trailing byte (",unpack("C",$c),")"
                if  1 == $len;
            return   if  0 == $len;
            ( $c, $prev{$fh} )= unpack "CC", $c;
            $c |= ($prev{$fh}&0xf)<<8;
            $prev{$fh} >>= 4;
            return $c;
        }
    }
}