So Dale, thanks for converting to C! Much more clear.
Leaving allpass filters aside for a moment;
When looking at the entirety of an interesting reverb loop(s), it is important to identify which elements represent feed back paths and which elements represent feed forward paths.
Each path will have a specific frequency response, and a gain which will determine how close to being flat in the frequency domain the element is. Typically, flatter is better. Typically, frequency peaks that are fat and round will sound better than those which are more needle like.
This understanding can give some insight into the behavior of a reverb of any topology, increasing the predictive nature of the design process and reducing the required amount of trial and error.
Of course reverb topologies are defined by many characteristics, but the purposeful decisions made as to how feed back and feed forward paths are used is a good place to start any design from.
-Casey