Allows to check if a user have defined a command.
Once an argument is installed the user may or may not define it. To define it simply means to use this argument as command line option. This function allows to check if an argument was used for the current command line option.
This function is expecting to receive the argument name to lookup not the short name. User may be using the short name to invoke the argument but the lookup is allways done by using the argument name.
- TRUE if argument was defined or FALSE if not.