code: public Signature getSignatureFrom(SignerIdentifier signer) {
          return signatures.stream()
                  .filter(signatureFrom(signer))
                  .findFirst()
                  .orElseThrow(() -> new IllegalArgumentException("Unable to find signature from this signer"));
      }
docs: Gets the signature from a given signer.
  
  @param signer an identifier referring to a signer of the job. It may be a personal identification number or
  contact information, depending of how the {@link PortalSigner signer} was initially created
  (using {@link PortalSigner#identifiedByPersonalIdentificationNumber(String, Notifications) personal identification number}<sup>1</sup>,
  {@link PortalSigner#identifiedByPersonalIdentificationNumber(String, NotificationsUsingLookup) personal identification number}<sup>2</sup>,
  {@link PortalSigner#identifiedByEmail(String) email address}, {@link PortalSigner#identifiedByMobileNumber(String) mobile number} or
  {@link PortalSigner#identifiedByEmailAndMobileNumber(String, String) both email address and mobile number}).
  <p>
  <sup>1</sup>: with contact information provided.<br>
  <sup>2</sup>: using contact information from a lookup service.
  </p>
  @throws IllegalArgumentException if the job response doesn't contain a signature from this signer