public final void injectSize(final int sizeoffset) {
        int size = 0;
        final int datasize = aboffset - sizeoffset - 1;
        int v = datasize;
        do {
            v >>= 7;
            size++;
        }
        while (v != 0);
        if (size > 1) { // doesn't fit -> shift the data after the placeholder
            System.arraycopy(ab, sizeoffset + 1, ab, sizeoffset + size, datasize);
        }
        aboffset = sizeoffset;
        writeVarLengthUnsigned(datasize);
        aboffset = sizeoffset + size + datasize;
    }
Just reserves a single byte and return it' offset.
Used in conjunction with injectVarLengthUnsigned
to efficiently write a size prefix

@return the offset of the placeholder
main/src/main/java/cgeo/geocaching/brouter/util/ByteDataWriter.java