b reduction and a renaming
Rule: during b reduction, if e2 contains any free variables that are bound in e1, then for each such free variable, it must be a-converted inside e1 BEFORE doing b -reduction!
( l f . ( l a . ( f a) ) ) ( l f . ( f a) ) ®a®
( l f . ( l b . ( f b) ) ) ( l f . ( f a) ) ®b®
( l b . (( l f . ( f a) ) b) )