I'm completely unfamiliar with the language but was able to read it from left-to-right very easily:

    var signal*: proc (a1: cint; a2: proc (a1: cint)): proc (a1: cint)

That reads to me as:

Variable signal is a pointer to a function that that takes two arguments (an int, and a function that takes an int), and returns a function that takes one argument (an int).

Of course, I'm assuming my interpretation is correct... perhaps it's not ;)

I agree, I prefer Nim's alternate syntax for this sort of stuff:

    var signal*: (cint,(cint)->void) -> (cint)->void