code: func (cgs *credentialGraphSet) Head(pe *pathexp.PathExp) (registry.CredentialGraph, error) {
  
  	gpe, err := pe.WithInstance("*")
  	if err != nil {
  		return nil, err
  	}
  
  	graphs, ok := cgs.graphs[gpe.String()]
  	if !ok {
  		return nil, nil
  	}
  
  	sort.Sort(graphSorter(graphs))
  	return graphs[0], nil
  }
docs: // Head returns the most recent version of a CredentialGraph that would contain
  // the given PathExp.