Basically this is to support "expression" like syntax such as people are used to in C, for example i++ and x=y are expressions with side effects in C and a lot of C idioms depend on such things. This kind of thing was originally not allowed but I personally found it clumsy to manually split out the imperative parts from the functional parts, so I taught the compiler how to do it for me :)

But those expressions also return things. Why can't they be gen? Why would I use a proc instead? It seems like I must be missing something, but I can't see it.