f (g x) = h (f x)

f (g (fix g)) = h (f (fix g))

f (fix g) = h (f (fix g))

f (fix g) = h (f (g (fix g)))

f (fix g) = h (h (f (fix g)))

f (fix g) = h (h (h (f (fix g))))

...

f (fix g) = fix h

 

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