[RF] extend testNaNPacker with demonstrations of arithmetic

I wrote this small test just to make sure arithmetic worked as expected on what at first I thought was a rather exotic class. I think in the end it also serves as a demonstration of how cool and useful this class is.

roofit/roofitcore/test/testNaNPacker.cxx

@@ -101,6 +101,43 @@ TEST(RooNaNPacker, CanPackStuffIntoNaNs)
       dumpFloats(rnp._payload);
 }
 
+// Demonstrate value preserving behavior after arithmetic on packed NaNs.
+TEST(RooNaNPacker, PackedNaNPreservedAfterArithmetic)
+{
+   RooNaNPacker rnp, rnp2;
+   rnp.setPayload(1.337f);
+   EXPECT_TRUE(rnp.isNaNWithPayload());
+
+   // multiply the packed NaN by 1 and use the result as rnp2's NaN with payload
+   rnp2._payload = 1. * rnp.getNaNWithPayload();
+   EXPECT_TRUE(rnp2.isNaNWithPayload());
+   EXPECT_EQ(rnp.getPayload(), rnp2.getPayload());
+
+   // multiply the packed NaN by -1
+   rnp2._payload = -1. * rnp.getNaNWithPayload();
+   EXPECT_TRUE(rnp2.isNaNWithPayload());
+   // minus signs on the NaN don't affect the payload
+   EXPECT_EQ(rnp.getPayload(), rnp2.getPayload());
+
+   // multiply the packed NaN by 4242
+   rnp2._payload = 4242. * rnp.getNaNWithPayload();
+   EXPECT_TRUE(rnp2.isNaNWithPayload());
+   // random multiplicative values on the NaN don't affect it all either
+   EXPECT_EQ(rnp.getPayload(), rnp2.getPayload());
+
+   // add 4242 to the packed NaN
+   rnp2._payload = rnp.getNaNWithPayload() + 4242.;
+   EXPECT_TRUE(rnp2.isNaNWithPayload());
+   // addition also has no effect, the NaN retains its bits
+   EXPECT_EQ(rnp.getPayload(), rnp2.getPayload());
+
+   // divide packed NaN by 1337, subtract 20 and take the modulo of 38 before calculating the sine of all this
+   rnp2._payload = std::sin(std::fmod((rnp.getNaNWithPayload() / 1337. - 20.), 38.));
+   EXPECT_TRUE(rnp2.isNaNWithPayload());
+   // nothing can harm the PackedNaN
+   EXPECT_EQ(rnp.getPayload(), rnp2.getPayload());
+}
+
 /// Fit a simple linear function, that starts in the negative.
 TEST(RooNaNPacker, FitSimpleLinear)
 {