Changes to TX21 per issue #270

Per issue #270 I have modified the spec to use ADD and SUBTRACT instead of Add/Update/Cancel. Logic for merging orders at the same price is described..
Need explanation and/or examples of how Add and Subtract work. If the original was 50 for sale & 100 desired. How do I specify that I want to add/subtract 20 for sale? What value goes into the amount desired field?

> ... when their unit prices are the same.

to what precision?