scope isNonEmptySingleZip_byMagicHeader.

static byte[] header = bytesFromHex("504B0304");

static bool isNonEmptySingleZip_byMagicHeader(byte[] data) {
  ret byteArrayStartsWith(data, header);
}

static bool isNonEmptySingleZip_byMagicHeader(File f) {
  ret isNonEmptySingleZip_byMagicHeader(loadBeginningOfBinaryFile(f, l(header)));
}