According to musicologists Paul Hegarty and Martin Halliwell, Bill Martin and Edward Macan authored major books about prog rock while "effectively accept the characterization of progressive rock offered by its critics. ... they each do so largely unconsciously." Academic John S. Cotner contests Macan's view that progressive rock cannot exist without the continuous and overt assimilation of classical music into rock. Author Kevin Holm-Hudson agrees that "progressive rock is a style far more diverse than what is heard from its mainstream groups and what is implied by unsympathetic critics."