allow randomization of part of the interaction

[juancarlosfarah]

If the interaction consists of n exchanges. We need to be able to:

1. Randomize the n exchanges.
2. Randomize within groups of the n exchanges (e.g., 1-4 randomized and 5-8 randomize so that 1-4 are always the first four, and 5-8 always the last four, albeit in different orders).
3. Randomize the full group. That is, sometimes it's 1-4 then 5-8 and sometimes 5-8 then 1-4.