1、reduce()
general form: reduce(f(), list[], int);
A. f() must has two parameter。eg:
f(x+y): x+y;
f(x*y): x*y;
B. list[] take the parameter
C. int is not necessary. it is the beginer for f().
def f(x, y ):
redurn x + y
reduce (f, [1,2,3,4,5], 100 or null)
