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The eta rewrite rule
An eta conversion lifts certain function calls out of a lambda convolute

A function f, which only passes its parameters on to another function e, can be substituted by e

(lambda(x) (e x)) <=> e   provided that x is not free in the expression e
Legal conversion:

Converted Expression

(lambda (x) (square x))
Illegal conversion:

Converted Expression

(lambda(x) ((lambda(y) (f x y)) x))
(lambda(y) (f x y))