A hybrid (Baddeley, Turner, Mateu and Bevan, 2012)
  is a point process model created by combining two or more
  point process models, or an interpoint interaction created by combining
  two or more interpoint interactions.
  
The function is.hybrid is generic, with methods for
  point process models (objects of class "ppm")
  and point process interactions
  (objects of class "interact").
  These functions return TRUE if the object x is a hybrid,
  and FALSE if it is not a hybrid.
Hybrids of two or more interpoint interactions
  are created by the function Hybrid.
  Such a hybrid interaction can then be fitted to point pattern data
  using ppm.