I wanted to calculate volume filling fractions and mass fractions of gas that met some criteria. Specifically, the fractions that have a certain metallicity. The _Action routine looked promising, but I couldn't figure out how to use it. I remember there was some talk about filters on the list, but I searched and found that it concerned the halo finder.
I might have reinvented the wheel, but this is how I did it.
It's basically _TotalQuantity with a filter. Also I wanted it to accept multiple limits to avoid re-reading the data for every filter.
Was there an easier way to do this?