Broken power law:

We examine a test source "multipower" defined thus:

<source name="multipower" flux="1.0">
    <
spectrum escale="GeV">
        <
particle name="gamma"> <power_law emin="0.01" emax="10.0" ebreak="0.5" gamma="2" gamma2="4"/>
        </
particle>
        <
solid_angle mincos="0" maxcos="1.0"/>
    </
spectrum>
</
source>

Note that, normally, the "power_law" element is declared with a minimum and maximum, and a single power law index "gamma.".  
The "ebreak" option allows for the two "gamma" variables to apply above and below the break.. If ebreak is not specified, it defaults to zero, for no break.

We run the standard flux tests on the above "multipower" source, and find a rate (after 10,000 events) of 37.6892, or 
about 1*2PI*6, where 1 is the rate (s-1 m-2 sr-1), the diffuse source is declared over 2PI steradians, and the 6 is the 
area of the detector circle, in m2.

Running a set of rootplots on the same source gives results like this:

as expected, the break in the power law happens at 0.5 GeV, with an index of 2 before and an index of 4 afterward.

It should be noted that this allows for other "bumpy" spectra outside of a simple hardening at higher energies.  For instance,
if the first energy index in "multipower" becomes a -2, then we have a spectrum that looks like this: