Original sentence: Progressive rock, sometimes shortened to prog rock or prog, is a form of rock music that evolved in the late 1960s and early 1970s, principally from psychedelic rock, and ultimately as a result of experimentation with electronic keyboard instruments, jazz-rock fusion, blues, and classical music
Edited sentence: Progressive rock, sometimes shortened to prog rock or prog, is a form of rock music that evolved in the late 1960s and early 1970s, principally from psychedelic rock, and ultimately as a result of experimentation with electronic keyboard instruments, jazz-rock fusion, blues, and mainly classical music