function _ctcValidityChecks(icu) {
  // '$$' i.e. "Double Dollar" is always invalid in ctc.
  const regex = /\$([^$]*?)\$/g;
  const matches = regex.exec(icu.message);
  if (Array.isArray(matches)) {
    matches.forEach(function(value) {
      if (!value) {
        throw new Error(`Ctc messages cannot contain double dollar: ${icu.message}`);
      }
    });
  }
}
Do some basic checks on a ctc object to confirm that it is valid. Future
ctc regression catching should go here.

@param {IncrementalCtc} icu the ctc output message to verify