In v2 of the protocol, the ESC document format now has three types:

1. non-anchor documents: { _id: F_S1(count) }

2. regular anchor documents: { _id: F_S1(0 || apos), value: F_S2(0 || count) }

3. null anchor documents: { _id: F_S1(0 || 0), value: F_S2(apos || cpos) }

Implement the helper functions in the ESCCollection class to generate and decrypt documents of the above formats.