code: def subset(self, selector):
          
          if isinstance(selector, (list, tuple)):
              return map(int, selector)
          selector = SELECTORS.get(selector, selector)
          mdtop = MDTrajTopology.from_openmm(self.handler.topology)
          return mdtop.select(selector)
docs: Returns a list of atom indices corresponding to a MDTraj DSL
          query. Also will accept list of numbers, which will be coerced
          to int and returned.