twitter/zipkin — pull request #260 by bmdhacks (2013-06-14 00:58:46+00:00)
Title: Make traces and spans monoids

Adding two spans with the same id will merge their annotations.
Adding two traces with the same id will merge their spans.
This is the basis for map/reduce jobs to coalesce spans into traces.
TODO - use the monoid for this behavior in the rest of the code.