Anyone can explain me this syntax?

Hi all...

>From Muenchian method of grouping, I always use something like this:

ROW[count(. | key('relacion_x_cobertura', REL_ID)[1]) = 1]

That always works, but I want to understand he reason of that syntax, so that if I don't have the possibility of doing copy & paste from other code, I could be able to write that expression by myself.

I know that instruction only returns the different elements contained in ROW. That difference is determined by REL_ID value.

But the actual questions are:

- Why the . (dot) is used? why if I omit it, it doesn't work (it returns all elements)?
- What's the meaning of the | (pipe)?
- What's the meaning of [1]? I have always used things like [FIELD_NAME=some_value]. I understand that perfectly, but what about placing only that number in the brackets? I tried by using [2] and it worked too... or, perhaps I was lucky?

Thanks a lot in advance
Jaime 

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