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 10.2.3 Expected Command Line Format
 -----------------------------------
 
 In order for `getopt-long' to correctly parse a command line, that
 command line must conform to a standard set of rules for how command
 line options are specified.  This subsection explains what those rules
 are.
 
    `getopt-long' splits a given command line into several pieces.  All
 elements of the argument list are classified to be either options or
 normal arguments.  Options consist of two dashes and an option name
 (so-called "long" options), or of one dash followed by a single letter
 ("short" options).
 
    Options can behave as switches, when they are given without a value,
 or they can be used to pass a value to the program.  The value for an
 option may be specified using an equals sign, or else is simply the next
 word in the command line, so the following two invocations are
 equivalent:
 
      $ ./foo.scm --output=bar.txt
      $ ./foo.scm --output bar.txt
 
    Short options can be used instead of their long equivalents and can
 be grouped together after a single dash.  For example, the following
 commands are equivalent.
 
      $ ./foo.scm --version --help
      $ ./foo.scm -v --help
      $ ./foo.scm -vh
 
    If an option requires a value, it can only be grouped together with
 other short options if it is the last option in the group; the value is
 the next argument.  So, for example, with the following option
 specification --
 
      ((apples    (single-char #\a))
       (blimps    (single-char #\b) (value #t))
       (catalexis (single-char #\c) (value #t)))
 
 -- the following command lines would all be acceptable:
 
      $ ./foo.scm -a -b bang -c couth
      $ ./foo.scm -ab bang -c couth
      $ ./foo.scm -ac couth -b bang
 
    But the next command line is an error, because `-b' is not the last
 option in its combination, and because a group of short options cannot
 include two options that both require values:
 
      $ ./foo.scm -abc couth bang
 
    If an option's value is optional, `getopt-long' decides whether the
 option has a value by looking at what follows it in the argument list.
 If the next element is a string, and it does not appear to be an option
 itself, then that string is the option's value.
 
    If the option `--' appears in the argument list, argument parsing
 stops there and subsequent arguments are returned as ordinary arguments,
 even if they resemble options.  So, with the command line
 
      $ ./foo.scm --apples "Granny Smith" -- --blimp Goodyear
 
 `getopt-long' will recognize the `--apples' option as having the value
 "Granny Smith", but will not treat `--blimp' as an option.  The strings
 `--blimp' and `Goodyear' will be returned as ordinary argument strings.
 

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