ParserDef-class         ParserDef S4 class
callbacks               Built-in callbacks for option definitions
defineOptions-package   Define and Parse Command Line Options
define_option           Function to define an option for argument
                        parsing
new_parser_def          Constructor of ParserDef class
parse_with_defs         Function to parse command line arguments with
                        ParserDef S4 object
summary.parsed_result   Summarize parsed_result S3 object
