http://en.wikipedia.org/wiki/Church_numerals#Church_pairs (example)

fst (pair a b)
\p.p (\x.\y.x) ((\x.\y.\z.z x y) a b)
\p.p (\x.\y.x) (\z.z a b)
???
(\z.z a b) (\x.\y.x)
(\x.\y.x) a b
a