The let alias trap with HashSet[string]

[Stefan_Salewski]

    import sets
    var h: HashSet[string]
    
    proc main =
      h.incl("Nim")
      let b = h
      h.incl("VLang")
      h = b
      echo h.contains("VLang")
    
    main()
    
    
    Run

The output is true.

But with "var b" it would be false. So it seems that "let b = h" creates just 
an alias.

I can now remember a similar case some years ago. But it is too hard to always 
have in mind that the code above with let is only an alias. With strings it 
seems to work like a real copy, and when all code is in global scope it seems 
to work also like a copy.
    
    
    Nim Compiler Version 1.1.1 [Linux: amd64]
    Compiled at 2020-03-26
    
    
    Run