code: func (r Account) GetNextInvoicePdf(documentCreateDate *datatypes.Time) (resp []byte, err error) {
  	params := []interface{}{
  		documentCreateDate,
  	}
  	err = r.Session.DoRequest("SoftLayer_Account", "getNextInvoicePdf", params, &r.Options, &resp)
  	return
  }
docs: // Return an account's next invoice in PDF format. The "next invoice" is what a customer will be billed on their next invoice, assuming no changes are made. Currently this does not include Bandwidth Pooling charges.