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: ebnf.go
    path of file: ./repos/participle
    
    the code of the file until where you have to start completion: package participle

import (
	"fmt"
	"strings"
)

// String returns the EBNF for the grammar.
//
// Productions are always upper cased. Lexer tokens are always lower case.
func (p *Parser[G]) String() string {
	return ebnf(p.typeNodes[p.rootType])
}

type ebnfp struct {
	name string
	out  string
}

func ebnf(n node) string {
	outp := []*ebnfp{}
	switch n.(type) {
	case *strct:
		buildEBNF(true, n, map[node]bool{}, nil, &outp)
		out := []string{}
		for _, p := range outp {
			out = append(out, fmt.Sprintf("%s = %s .", p.name, p.out))
		}
		return strings.Join(out, "\n")

	default:
		out := &ebnfp{}
		buildEBNF(true, n, map[node]bool{}, out, &outp)
		return out.out
	}
}

func buildEBNF(root bool, n node, seen map[node]bool, p *ebnfp, outp *[]*ebnfp) {
	switch n := n.(type) {
	case *disjunction:
		if !root {
			p.out += "("
		}
		for i, next := range n.nodes {
			if i > 0 {
				p.out += " | "
			}
			buildEBNF(false, next, seen, p, outp)
		}
		if !root {
			p.out += ")"
		}

	case *union:
		name := strings.ToUpper(n.typ.Name()[:1]) + n.typ.Name()[1:]
		if p != nil {
			p.out += name
		}
		if seen[n] {
			return
		}
		p = &ebnfp{name: name}
		*outp = append(*outp, p)
		seen[n] = true
		for i, next := range n.disjunction.nodes {
			if i > 0 {
				p.out += " | "
			}
			buildEBNF(false, next, seen, p, outp)
		}

	case *custom:
		name := strings.ToUpper(n.typ.Name()[:1]) + n.typ.Name()[1:]
		p.out += name

	case *strct:
		name := strings.ToUpper(n.typ.Name()[:1]) + n.typ.Name()[1:]
		if p != nil {
			p.out += name
		}
		if seen[n] {
			return
		}
		seen[n] = true
		p = &ebnfp{name: name}
		*outp = append(*outp, p)
		buildEBNF(true, n.expr, seen, p, outp)

	case *sequence:
		group := n.next != nil && !root
		if group {
			p.out += "("
		}
		for n != nil {
			buildEBNF(false, n.node, seen, p, outp)
			n = n.next
			if n != nil {
				p.out += " "
			}
		}
		if group {
			p.out += ")"
		}

	case *parseable:
		p.out += n.t.Name()

	case *capture:
		buildEBNF(false, n.node, seen, p, outp)

	case *reference:
		p.out += "<" + strings.ToLower(n.identifier) + ">"

	case *negation:
		p.out += "~"
		buildEBNF(false, n.node, seen, p, outp)

	case *literal:
		p.out += fmt.Sprintf("%q", n.s)

	case *group:
		if child, ok := n.expr.(*group); ok && child.mode == groupMatchOnce {
			buildEBNF(false, child.expr, seen, p, outp)
		}
    
func buildEBNF(root bool, n node, seen map[node]bool, p *ebnfp, outp *[]*ebnfp) {
	switch n := n.(type) {
	case *disjunction:
		if !root {
			p.out += "("
		}
		for i, next := range n.nodes {
			if i > 0 {
				p.out += " | "
			}
			buildEBNF(false, next, seen, p, outp)
		}
		if !root {
			p.out += ")"
		}

	case *union:
		name := strings.ToUpper(n.typ.Name()[:1]) + n.typ.Name()[1:]
		if p != nil {
			p.out += name
		}
		if seen[n] {
			return
		}
		p = &ebnfp{name: name}
		*outp = append(*outp, p)
		seen[n] = true
		for i, next := range n.disjunction.nodes {
			if i > 0 {
				p.out += " | "
			}
			buildEBNF(false, next, seen, p, outp)
		}

	case *custom:
		name := strings.ToUpper(n.typ.Name()[:1]) + n.typ.Name()[1:]
		p.out += name

	case *strct:
		name := strings.ToUpper(n.typ.Name()[:1]) + n.typ.Name()[1:]
		if p != nil {
			p.out += name
		}
		if seen[n] {
			return
		}
		seen[n] = true
		p = &ebnfp{name: name}
		*outp = append(*outp, p)
		buildEBNF(true, n.expr, seen, p, outp)

	case *sequence:
		group := n.next != nil && !root
		if group {
			p.out += "("
		}
		for n != nil {
			buildEBNF(false, n.node, seen, p, outp)
			n = n.next
			if n != nil {
				p.out += " "
			}
		}
		if group {
			p.out += ")"
		}

	case *parseable:
		p.out += n.t.Name()

	case *capture:
		buildEBNF(false, n.node, seen, p, outp)

	case *reference:
		p.out += "<" + strings.ToLower(n.identifier) + ">"

	case *negation:
		p.out += "~"
		buildEBNF(false, n.node, seen, p, outp)

	case *literal:
		p.out += fmt.Sprintf("%q", n.s)

	case *group:
		if child, ok := n.expr.(*group); ok && child.mode == groupMatchOnce {
			buildEBNF(false, child.expr, seen, p, outp)
		} else if child, ok := n.expr.(*capture); ok {
			if grandchild, ok := child.node.(*group); ok && grandchild.mode == groupMatchOnce {
				buildEBNF(false, grandchild.expr, seen, p, outp)
			} else {
				buildEBNF(false, n.expr, seen, p, outp)
			}
		} else {
			buildEBNF(false, n.expr, seen, p, outp)
		}
		switch n.mode {
		case groupMatchNonEmpty:
			p.out += "!"
		case groupMatchZeroOrOne:
			p.out += "?"
		case groupMatchZeroOrMore:
			p.out += "*"
		case groupMatchOneOrMore:
			p.out += "+"
		case groupMatchOnce:
		}

	case *lookaheadGroup:
		if !n.negative {
			p.out += "(?= "
		} else {
			p.out += "(?! "
		}
		buildEBNF(true, n.expr, seen, p, outp)
		p.out += ")"

	default:
		panic(fmt.Sprintf("unsupported node type %T", n))
	}
}