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The beta rewrite rule
A beta conversion tells how to evaluate a function call

An application of a function can be substituted by the function body, in which formal parameters are substituted by the corresponding actual parameters

Legal conversions:
Expression

 Converted Expression

((lambda(x) (f x)) a)
 
(f a)
((lambda(x y) (* x (+ x y))) (+ 3 4) 5)
 
(* 7 (+ 7 5))
((lambda(x y) (* x (+ x y))) (+ 3 4) 5)
 
(* (+ 3 4) (+ (+ 3 4) 5))