code: func NewErrFilePosf(file string, line, col int, format string, args ...interface{}) error {
  	return &errFilePos{
  		error: fmt.Errorf(format, args...),
  		file:  file,
  		line:  line,
  		col:   col,
  	}
  }
docs: // NewErrFilePosf creates an error conforming to the ErrFilePos interface.