Woody Guthrie docu: why is Arlo so conspicuously absent?

Great documentary, a fair and balanced overview of Woody Guthrie's life, music and influence. Fantastic interviews with Both of Woody's wives, his sister, his musical compatriot Pete Seeger, and his daughter Nora. Conspicuously missing, however, is Arlo. He is mentioned once by name and seen once in archival footage from the sixties, visiting Woody in the hospital (it may not even be him, as it is shot from the back!)

What's up with that? Anybody know?

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...I was cured, all right.