public function wasCalledOnce(): bool
    {
        $callsReceived = count($this->invocations);
        if (1 !== $callsReceived) {
            throw new CallAmountViolation(sprintf(
                    '%s was expected to be called once, but actually %s.',
                    $this->invocations->name(),
                    1 < $callsReceived ?
                        'called ' . $callsReceived . ' time(s)' :
                        'never called'

            ));
        }

        return true;
    }