void set(int idx, int byteOffset, int firstChild, int extBits) {
        data[idx] = (long)(byteOffset & 0x7fff_ffff) << 33 |
            (long)(firstChild & 0x7fff_ffff) << 2 |
            extBits & 0x3;
        ++used;
    }
Light-weight index structure describing the layout of a Slime value
encoded in binary format.
vespajlib/src/main/java/com/yahoo/slime/DecodeIndex.java