==> probability/coupon.p <==
There is a free gift in my breakfast cereal. The manufacturers say that
the gift comes in four different colors, and encourage one to collect
all four (& so eat lots of their cereal). Assuming there is an equal
chance of getting any one of the colors,  what is the expected number
of boxes I must consume to get all four?  Can you generalise to n
colors and/or unequal probabilities?