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

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 * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
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'use strict';

const jsonld = require('jsonld');
const jsigs = require('jsonld-signatures');
const CredentialIssuancePurpose = require('./CredentialIssuancePurpose');
const defaultDocumentLoader = jsigs.extendContextLoader(
  require('./documentLoader'));

// Z and T can be lowercase
// RFC3339 regex
const dateRegex = new RegExp('^(\\d{4})-(0[1-9]|1[0-2])-' +
    '(0[1-9]|[12][0-9]|3[01])T([01][0-9]|2[0-3]):' +
    '([0-5][0-9]):([0-5][0-9]|60)' +
    '(\\.[0-9]+)?(Z|(\\+|-)([01][0-9]|2[0-3]):' +
    '([0-5][0-9]))$', 'i');

module.exports = {
  issue,
  verify,
  createPresentation,
  CredentialIssuancePurpose,
  defaultDocumentLoader,