Signal Transition Graphs (STGs) are used to specify the ordering of positive and negative edges. This is actually a form of Petri Net but without the places. The STG for one stage of an event FIFO is depicted in the next figure. The dotted lines represent the response from the environment (effect of output to input changes) and solid lines represent internal response (effect of input to output changes).

Here is the STG for a C element that might be used in an asynchronous FIFO: