Hopefully simple Nim syntax question

[PMunch]

@aEverr is correct. The actual error here is that `toHeapQueue` requires a 
variable (something it can modify, so a `let` won't work, only `var`). 
Unfortunately values that you only pass in to a procedure aren't considered 
variable by the Nim compiler for some reason. So you have to first assign it to 
a variable, or use `dup` from the `sugar` module.