docs: function eifelerRegelAppliesToNumber(number) {
      number = parseInt(number, 10);
      if (isNaN(number)) {
          return false;
      }
      if (number < 0) {
          // Negative Number --> always true
          return true;
      } else if (number < 10) {
          // Only 1 digit
          if (4 <= number && number <= 7) {
              return true;
          }
          return false;
      } else if (number < 100) {
          // 2 digits
          var lastDigit = number % 10, firstDigit = number / 10;
          if (lastDigit === 0) {
              return eifelerRegelAppliesToNumber(firstDigit);
          }
          return eifelerRegelAppliesToNumber(lastDigit);
      } else if (number < 10000) {
          // 3 or 4 digits --> recursively check first digit
          while (number >= 10) {
              number = number / 10;
          }
          return eifelerRegelAppliesToNumber(number);
      } else {
          // Anything larger than 4 digits: recursively check first n-3 digits
          number = number / 1000;
          return eifelerRegelAppliesToNumber(number);
      }
  }