Nested set tricks

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Check how many descendents node i has:

For every descendent that i has, two numbers will be required (left (lft) boundary and right (rgt) boundary). Therefore the number of descendents of a node will be half of the difference between its boundaries.

   x = (i.rgt-i.lft)/2

There seems to be an instance method for doing exactly that: (see lib/nested_set/descendants.rb)

i.descendants_count