How to use the ripple-keypairs.sign function in ripple-keypairs

if (this.hasMultiSigners()) {
    return this;
  }

  const prev_sig = this.tx_json.TxnSignature;
  delete this.tx_json.TxnSignature;
  const hash = this.signingHash();

  // If the hash is the same, we can re-use the previous signature
  if (prev_sig && hash === this.previousSigningHash) {
    this.tx_json.TxnSignature = prev_sig;
    return this;
  }

  const keypair = deriveKeypair(secret || this._secret);
  this.tx_json.TxnSignature = sign(new Buffer(this.signingData(), 'hex'),
    keypair.privateKey);
  this.previousSigningHash = hash;

  return this;
};

function computeSignature(tx, privateKey, signAs) {
  const signingData = signAs ?
    binary.encodeForMultisigning(tx, signAs) : binary.encodeForSigning(tx);
  return rippleKeypairs.sign(signingData, privateKey);
}

function computeSignature(tx: Object, privateKey: string, signAs: ?string) {
  const signingData = signAs ?
    binary.encodeForMultisigning(tx, signAs) : binary.encodeForSigning(tx)
  return keypairs.sign(signingData, privateKey)
}

function signPaymentChannelClaim(
  channel: string,
  amount: string,
  privateKey: string
): string {
  validate.signPaymentChannelClaim({channel, amount, privateKey})

  const signingData = binary.encodeForSigningClaim({
    channel: channel,
    amount: xrpToDrops(amount)
  })
  return keypairs.sign(signingData, privateKey)
}

function computeSignature(tx: object, privateKey: string, signAs?: string) {
  const signingData = signAs
    ? binaryCodec.encodeForMultisigning(tx, signAs)
    : binaryCodec.encodeForSigning(tx)
  return keypairs.sign(signingData, privateKey)
}