code: func (f *file) errorf(n ast.Node, confidence float64, args ...interface{}) *Problem {
  	pos := f.fset.Position(n.Pos())
  	if pos.Filename == "" {
  		pos.Filename = f.filename
  	}
  	return f.pkg.errorfAt(pos, confidence, args...)
  }
docs: // The variadic arguments may start with link and category types,
  // and must end with a format string and any arguments.
  // It returns the new Problem.