Readability is a subjective question, but IMHO assigning relation_type to type won't improve readability, because the variable name inside the function body won't match the name of function argument, and so to understand where this variable came from developer will have to know the whole function body, not only function signature and the line he's interested in.

I don't think that `relation_type` is that annoying to type, and it reads better than 'rtype'. But I'm fine with rtype or reltype or typ if you think they are better - you're writing this code, so you're the one to paint this bike shed :)