code: func Where(l *State, level int) {
  	if f, ok := Stack(l, level); ok { // check function at level
  		ar, _ := Info(l, "Sl", f) // get info about it
  		if ar.CurrentLine > 0 {   // is there info?
  			l.PushString(fmt.Sprintf("%s:%d: ", ar.ShortSource, ar.CurrentLine))
  			return
  		}
  	}
  	l.PushString("") // else, no information available...
  }
docs: // Where pushes onto the stack a string identifying the current position of
  // the control at level in the call stack. Typically this string has the
  // following format:
  //   chunkname:currentline:
  // Level 0 is the running function, level 1 is the function that called the
  // running function, etc.
  //
  // This function is used to build a prefix for error messages.