code: function isValidIdentifierChar(ch, first) {
      // check directly for ASCII
      if (ch <= "\u007F") {
          if ((ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z') || ch === '$' || ch === '_' ||
              (!first && (ch >= '0' && ch <= '9'))) {
              return true;
          }
          return false;
      }
  
      // create an object to test this in
      var x = {};
      x["x"+ch] = true;
      x[ch] = true;
  
      // then use eval to determine if it's a valid character
      var valid = false;
      try {
          valid = (Function("x", "return (x." + (first?"":"x") + ch + ");")(x) === true);
      } catch (ex) {}
  
      return valid;
  }
docs: /*
  Since JavaScript provides no convenient way to determine if a
  character is in a particular Unicode category, we use
  metacircularity to accomplish this (oh yeaaaah!)