private boolean fileOk () throws IOException {
        byte[] buf = new byte[CART_ID.length];
        source.seek(0);
        source.readFully(buf);
        for (int i = 0; i < buf.length; i++) if (buf[i]!=CART_ID[i]) return false;
        return true;
    }
Implementation of the GWC cartridge format.
<p>
This class handles reading the GWC file format, extracting information
from its header and reading individual files stored inside.
main/src/main/java/cgeo/geocaching/wherigo/openwig/formats/CartridgeFile.java