[Tutor] Help understanding part of the tutorial

[Guess Who? Me]

http://www.honors.montana.edu/~jjc/easytut/easytut/node9.html
contains this function:

def mult(a,b):
    if b == 0:
        return 0
    rest = mult(a,b - 1)
    value = a + rest
    return value

print "3*2 = ",mult(3,2)

My question is that I don't get how mult(a,b-1) returns a value - can 
somebody please explain? And it still works properly if you make a==0 - but 
there is no "if a==0: return 0" statement. Any help would be appreciated.

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