2. 0<n
3.
f:(
(n-1)
), m:(n-1).
3. sum(f(x) | x < n-1) = f(m)+sum(if x=
m
0 else f(x) fi | x < n-1)
4. f :
n
5.
m:(n-1). sum(f(x) | x < n-1) = f(m)+sum(if x=m 0 else f(x) fi | x < n-1)
6. m :
n
7.
m = n-1
8. sum(f(x) | x < n-1) = f(m)+sum(if x=
m 0 else f(x) fi | x < n-1)
if n=0 0 else (
x,n. n+f(x))(n-1,sum(f(x) | x < n-1)) fi
=
f(m)+if n=0 0
f(m)+else (x,n. n+if x=m 0 else f(x) fi)
f(m)+else (n-1
f(m)+else ,sum(if x=m 0 else f(x) fi | x < n-1)) fi
