add PRED example

[brycec]

add the predecessor function (n-1) as an example.

PRED := λn.λf.λx.n (λg.λh.h (g f)) (λu.x) (λu.u)

de brujin notation: ###(((3 ##(1 (2 4))) #2) #1)

example of (PRED 4): (###(((3 ##(1 (2 4))) #2) #1) ##(2 (2 (2 (2 1)))))