ceph/src/boost/libs/program_options/doc/rationale

   1
   2 We could either implement simple chaining for variable maps, or
   3 implement generic composition classes. The former was choosen,
   4 mostly because of simplicity.
   5
   6 There were two implementation approaches for multiple option
   7 occurrences in options_and_arguments. First is store them
   8 separately. The advantage is that it's easy to obtain all
   9 occurrences before certain position on command line. The
  10 disadvantage is that we cannot return a reference to
  11 vector<vector<string> > in get_all_values. It was considered
  12 that if support for position-dependent options is to be
  13 added, then we're be mostly interested in occurrences of
  14 a single option that were before some point. That's possible
  15 with vector<vector<string> > storage.