Recursion

In Wall, any function can refer to itself in its definition. Here is the famous McCarthy 91 function written in Wall.

w> mc = fun [int] (? (< 100 a0) (- a0 10) (mc (mc (+ a0 11))))
w> mc -100
91
w> mc 5
91
w> mc 90
91
w> mc 1000
990