You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: ipldsch_satisfaction.go
    path of file: ./repos/sealer/vendor/github.com/ipld/go-codec-dagpb
    
    the code of the file until where you have to start completion: package dagpb

// Code generated by go-ipld-prime gengo.  DO NOT EDIT.

import (
	ipld "github.com/ipld/go-ipld-prime"
	"github.c
    
func (na *_PBLink__ReprAssembler) AssignNull() error {
	switch *na.m {
	case allowNull:
		*na.m = schema.Maybe_Null
		return nil
	case schema.Maybe_Absent:
		return mixins.MapAssembler{TypeName: "dagpb.PBLink.Repr.Repr"}.AssignNull()
	case schema.Maybe_Value, schema.Maybe_Null:
		panic("invalid state: cannot assign into assembler that's already finished")
	case midvalue:
		panic("invalid state: cannot assign null into an assembler that's already begun working on recursive structures!")
	}
	panic("unreachable")
}