groups and background

[Dar Scott]

Let's see if I got this right.

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A group may have its backgroundBehavior property set to true.  Such a 
group introduces interesting paths on any card it is on.  This group is 
also placed on new cards under some conditions.  The card/stack property 
backgroundNames lists all groups with this property set to true for the 
specified card or stack.

Groups may be shared.   "The number of backgrounds (of a stack)" is the 
number of sharable groups in the stack; shared groups are counted only 
once.  (The number of backgrounds of a card is 0.)  All groups are 
sharable except nested groups.  The property groupNames is a card 
property that lists all sharable groups on the card.  There is no such 
property for stacks.

"The number of groups" [of a specified card] is the total number of 
groups on the card including shared and nested.  The number of groups of 
a stack is the total number of groups on the current card.  Shared 
groups are counted just once.

The expression "group <groupref>" refers to a group on the current or 
specified card.  The expression "background <groupref> refers to any 
group in the current or specified stack (including nested groups).  The 
layer number (if used) for the background reference refers to the 
creation order.

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Is this right?  Is this complete?  Is the word group or background used 
in any other way?

Dar Scott