code: def w(name, parallel, workflow, internal):
      
      Workflow = collections.namedtuple("Workflow", "name parallel workflow internal")
      return Workflow(name, parallel, workflow, internal)
docs: A workflow, allowing specification of sub-workflows for nested parallelization.
  
      name and parallel are documented under the Step (s) function.
      workflow -- a list of Step tuples defining the sub-workflow
      internal -- variables used in the sub-workflow but not exposed to subsequent steps