@mashingan
No, it's not about number of arguments. See that:
proc fun(x,y: int) = echo x+y
proc gun(x,y: int): int = x+y
fun 1, 2 # works fine
let x = gun 1, 2 # doesn't compile
let x = gun 1: 2 # compiles (!), although it looks weird
Just like GULPF said, it's possible to use multiple arguments but only for statements. So yes, in practice it's probably better to use it in single-argument cases as Nim is becoming more and more expression-oriented (luckily!).
