The chapter raises the question of the semantics of

    {
      var a = "outer";
      {
        var a = a;
      }
    }

and

    {
      var a = a;
    }

. Some programming languages have the idea of "let rec" versus "nonrecursive let." In Haskell, `let` is recursive, and because Haskell is lazy, `let a = a` is a nonterminating computation. In OCaml, `let` is non-recursive and `let rec` is recursive, and only certain recursive definitions are allowed. (You can do `let rec a = 0::a`, but not `let rec a = a`.)

Scheme has `let`, where none of the variables are in scope for any definition, `let*`, where each variable is in scope in the successive definitions, and `letrec`, where all of the variable are in scope for all definitions. According to http://www.r6rs.org/final/r6rs.pdf, Scheme initializes letrec'd variables to a "black hole."