Question: How is this Basisgrid procedure programmed?

I did already some programmingexercises , but this sort of programming example i did not encounter in the programmingbook
How it is programmed in general lines ?
I do have a lot of small procedures to learn from

But it is too much asked here on the forum to explain all ins and outs for this procedure, because it is not that simple. 
 

Download Basisgrid_procedure.mw

 

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