Tests one argument. This method is invoked from parseArgument. It is abstract in the superclass MainCmd
and must be overwritten from the user.
:TODO: user, test and evaluate the content of the argument string
or test the number of the argument and evaluate the content in dependence of the number.
Checks the cmdline arguments relation together.
If there is an inconsistents, a message should be written. It may be also a warning.
:TODO: the user only should determine the specific checks, this is a sample.