Shared arguments across subcommands

Is something like [this](https://stackoverflow.com/questions/27236570/command-line-argument-parsing-in-haskell-that-supports-shared-arguments) possible, with shared arguments across subcommands?

Start with the linked Stack Overflow example, then trace how optparse-generic represents subcommands and their arguments. Determine whether shared arguments are supported by the current design and define the expected behavior before proposing an implementation or tests.

Written by the indexing model from the issue text.