code: func NewContextsWriter(filename string) (*ContextsWriter, error) {
  	file, err := codegen.SourceFileFor(filename)
  	if err != nil {
  		return nil, err
  	}
  	return &ContextsWriter{
  		SourceFile: file,
  		Finalizer:  codegen.NewFinalizer(),
  		Validator:  codegen.NewValidator(),
  	}, nil
  }
docs: // NewContextsWriter returns a contexts code writer.
  // Contexts provide the glue between the underlying request data and the user controller.