named list; containing the following functions:
flag_test, flag_to_name and name_to_flag
If style is not specified, opt_style gets the current optigrab
style. If style is provided, it must be a named list containing
three functions flag_test, flag_to_name and
name_to_flag.
Value
If style is not provided, returns a list of styles, otherwise used
for the side-effect of setting the option
flag_test
Accepts a character vector and returns a logical vector indicating whether
the elements are flags.
flag_to_name
Accepts a character vector of flags and turns them into variable names,
usually by stripping delimiters that indicate that they are flags
name_to_flag
Accepts a character vector of names and transforms them into the flags that
would appear on the command line. This is used by opt_grab().