code: func (t *transformer) transformRegisterServerFuncs(s *service) {
  	registrarName := ast.NewIdent("Registrar")
  	var registrarType ast.Expr = registrarName
  	if !t.inPRPCPackage {
  		registrarType = &ast.SelectorExpr{serverPrpcPkg, registrarName}
  	}
  	s.registerServerFunc.Params.List[0].Type = registrarType
  }
docs: // transformRegisterServerFuncs finds RegisterXXXServer functions and
  // checks its first parameter type to prpc.Registrar.
  // Returns true if modified ast.