code: def create(cls, ltc, event_class, event_types, ebins):
          
  
          evals = np.sqrt(ebins[1:] * ebins[:-1])
          exp = np.zeros((len(evals), ltc.hpx.npix))
          for et in event_types:
              aeff = create_aeff(event_class, et, evals, ltc.costh_center)
              exp += np.sum(aeff.T[:, :, np.newaxis] *
                            ltc.data[:, np.newaxis, :], axis=0)
  
          hpx = HPX(ltc.hpx.nside, ltc.hpx.nest,
                    ltc.hpx.coordsys, ebins=ebins)
          return cls(exp, hpx)
docs: Create an exposure map from a livetime cube.  This method will
          generate an exposure map with the same geometry as the
          livetime cube (nside, etc.).
  
          Parameters
          ----------
          ltc : `~fermipy.irfs.LTCube`
              Livetime cube object.
  
          event_class : str
              Event class string.
  
          event_types : list
              List of event type strings, e.g. ['FRONT','BACK'].
  
          ebins :  `~numpy.ndarray`
              Energy bin edges in MeV.