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by Harrison Reiser

Internal Rate of Return (IRR ???
http://en.wikipedia.org/wiki/Internal_rate_of_return) is a common financial
metric, used by investment firms to predict the profitability of a company or
project. Finding the IRR of a company amounts to solving for it in the equation
for Net Present Value (NPV ??? http://en.wikipedia.org/wiki/Net_present_value),
another valuable decision-making metric:

N      C_t
NPV =  Σ  ------------
t=0 (1 + IRR)**t

This week's quiz is to calculate the IRR for any given variable-length list of
numbers, which represent yearly cash flows, the C_t's in the formula above: C_0,
C_1, etc. (C_0 is typically a negative value, corresponding to the initial
investment into the project.) From the example in the Wikipedia article
(http://en.wikipedia.org/wiki/Internal_rate_of_return), for instance, you should
be able to produce a rate of 17.09% (to four decimal places, let's say) from
this or a similar command:

irr([-100,+30,+35,+40,+45])
=> 0.1709...

Keep in mind that an IRR greater than 100% is possible. Extra credit if you can
also correctly handle input that produces negative rates, disregarding the fact
that they make no sense.

```