describes an optional parameter
This parameter may appear zero or once in the parameter list.
| std::MANDATORY_PARAMETER | describes a mandatory parameter |
| std::SOME_PARAMETERS | describes list of zero or more parameters |
| std::MANY_PARAMETERS | describes list of one or more parameters |
| std::VALUED_OPTION | describes an option with an associated value |
| std::MULTI_VALUED_OPTION | describes an option with an associated list of values |