reference historical OHLC x bars back for robot vs. indicator

Greetings all -

Very new to cTrader, so apologies for the questions — I have spent some time reviewing API and examining cTrader reference and community indicators and robot code to familiarize myself with the proper code syntax. Have built a few indicators, etc.

However, it seems like some of the examples in the guides are not in sync with the current API, and there are several ways to do things, and none explicit (at least to me…) so a bit confused, and some guidance will be very much appreciated. I am used to finding my own answers in the docs, but not having any luck. Maybe it is just me…So, please:

Referencing bars

A - is there one way to do it that is correct syntax for indicators, but different syntax to reference bars back for robots? If so, what are those methods, and is there any other piece of code necessary to make it work?

In robot for ex., I put code in OnBar, since OHLC needs a finished bar to reference. Does this make sense?

I can see it requires MarketSeries and Last, or LastValue.

Either square (or round) brackets, and an int to hold the index, or (not, if no index). 

Some examples reference using (1) (2), but some others reference (-2), etc.

(Arrays in C# usually count from 0, so I was even wondering about how that fit in…)

Rather than me post code to all the things I have tried to work this out myself by printing variations to the log and hand checking the history on the charts - if anyone can set me straight or point me to documentation that isn't contradicted elsewhere, or deprecated in the API, I would be most grateful.

so...For robot:

Create variables for OHLC of last closed bar 

Create variables for OHLC of bar before that

Thanks so very much