title: what is the advantage of using negative enums
body: <p>In the kaleidoscope parser / AST example at <a href="http://llvm.org/docs/tutorial/LangImpl2.html#code" rel="nofollow noreferrer">LLVM</a>, the enum is given all negative values.  Why the minus sign ? <pre>enum Token {
    tok_eof = -1,
    // commands
    tok_def = -2, tok_extern = -3,
    // primary
    tok_identifier = -4, tok_number = -5
  };</pre></p>