Compiler time type checking in Nim

[MisterDrgn]

It looks like my understanding was pretty far off. For example, the code below 
works and is checked at compile time.

Maybe my issue is with finding the right documentation on concepts. To write 
the example below, I had to use the RFC: 
<https://github.com/nim-lang/RFCs/issues/168>

Thanks for the help.
    
    
    type
        Addable = concept
          proc `+`(x, y: Self): Self
    
    proc h[T: Addable](x: T): T = x + 1
    
    var y = h(4)
    
    echo y
    
    Run