If it isn't based on rock in the first place then it can't be Prog Rock - It don't get no simpler than that Hoss!
Avantgarde jazz, jazzy modern classical, folk jazz, afro/asian/latin jazz, electronic soundscapes, avantgarde classical, new age electronica, new age folk, Tape Music, Found Sound and so on - none of them are rock, so none of them are Prog Rock.
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