I'm trying to read this recent paper (http://arxiv.org/abs/1507.07745) but I don't understand the definition of an "operation" in the paper. It is defined as a set of events on the first page, but I don’t know what an event is in this context. Looks like it’s some kind of function but I can’t verify that or tell any details. E.g. is it really a function, if so is it a linear transformation, what kind of spaces does it map between, …? An example would be great as well. Anyone know if there’s a link with a simple example?