How to use the jsonld-signatures.purposes function in jsonld-signatures

async function _generateCredential() {
  const mockCredential = jsonld.clone(mockData.credentials.alpha);
  const {authenticationKey, documentLoader} = await _generateDid();
  const {Ed25519Signature2018} = jsigs.suites;
  const {AuthenticationProofPurpose} = jsigs.purposes;
  testLoader.addLoader(documentLoader);
  const credential = await jsigs.sign(mockCredential, {
    compactProof: false,
    documentLoader: testLoader.documentLoader.bind(testLoader),
    suite: new Ed25519Signature2018({key: authenticationKey}),
    purpose: new AuthenticationProofPurpose({
      challenge: 'challengeString'
    })
  });
  return {credential, documentLoader};
}

const getPublicKeyFromDIDDoc = require("./getPublicKeyFromDIDDoc");
const OpenPgpSignature2019 = require("@transmute/openpgpsignature2019");
const wrappedDocumentLoader = require("./wrappedDocumentLoader");

const jsigs = require("jsonld-signatures");
const { Ed25519Signature2018 } = jsigs.suites;
const { AssertionProofPurpose } = jsigs.purposes;
const { Ed25519KeyPair } = require("crypto-ld");

const verifyWithResolver = async (signedData, resolver) => {
  const verificationMethod = signedData.proof.verificationMethod
    ? signedData.proof.verificationMethod
    : signedData.proof.creator;
  const doc = await resolver.resolve(verificationMethod);

  const didPublicKey = doc.publicKey.find(k => {
    return k.id == verificationMethod;
  });

  if (didPublicKey.type === "Ed25519VerificationKey2018") {
    const result = await jsigs.verify(signedData, {
      documentLoader: wrappedDocumentLoader({
        //args that are needed in the wrapper.. such as zcaps

/*!
 * Copyright (c) 2019 Digital Bazaar, Inc. All rights reserved.
 */
'use strict';
const {AssertionProofPurpose} = require('jsonld-signatures').purposes;

/**
 * Creates a proof purpose that will validate whether or not the verification
 * method in a proof was authorized by its declared controller for the
 * proof's purpose.
 *
 * Required params:
 * - `controller`
 */
class CredentialIssuancePurpose extends AssertionProofPurpose {
}

module.exports = CredentialIssuancePurpose;