What happened to MathSum?

[antiseptic]

I want to calculate the sum of part of an array of x number of periods.  MathSum seemed to be what I was looking for, but while it appears in online documentation, it does not appear to be in MetaTrader 5 or the local reference (Help>MQL5 Reference).

Is there another function? Or am I just going to have to write my own?

[Alain Verleyen]

MathSum() is a function from Standard library, you need to add this include in your code to use it :

#include <Math\Stat\Math.mqh>

Unfortunately the Standard library documentation is Spartan.