Fix build break due to generalized type parameter bounds
Fix build break due to generalized type parameter bounds

birkenfeld/docopt.rs,zsiciarz/docopt.rs,birkenfeld/docopt.rs,birkenfeld/docopt.rs,zsiciarz/docopt.rs,alexcrichton/docopt.rs,brandonson/docopt.rs,brandonson/docopt.rs,alexcrichton/docopt.rs,brandonson/docopt.rs,zsiciarz/docopt.rs