code: def from_string(vc):
          
          try:
              # Note: this means all version controls must have
              # a title naming convention (!)
              vc = globals()[vc.title()]
              assert(issubclass(vc, VersionControl))
              return vc
          except (KeyError, AssertionError):
              raise NotImplementedError("Unknown version control system.")
docs: Return the VersionControl superclass from a string, for example
          VersionControl.from_string('git') will return Git.