code: @Nonnull
    protected static EValidity validateMessage (@Nonnull final char [] aChars)
    {
      ValueEnforcer.notNull (aChars, "Chars");
  
      for (final char c : aChars)
        if (c < '0' || c > '9')
          return EValidity.INVALID;
      return EValidity.VALID;
    }
docs: Validates a UPC/EAN/GTIN/GLN message.
  
  @param aChars
  the chars to validate
  @return {@link EValidity#VALID} if the msg is valid,
  {@link EValidity#INVALID} otherwise.