Understanding Nim compiler

[spip]

Is there a way to understand and trace Nim compiler? For instance, this [piece 
of code](https://play.nim-lang.org/#ix=2rpf) does not compile, while if you 
replace `template` by `proc`, it does. Is it possible to trace the compiler 
parsing the code and understand the difference between both behaviours? One one 
would start looking at the compiler internals? Is the [wiki 
page](https://github.com/nim-lang/Nim/wiki/Debugging-the-compiler) still the 
best way to jump into debugging the compiler?