st: number generation

[Tyler Frazier <tyfrazier@gmail.com>]

I would like to generate a method to number buildings in sequence, but
restart the sequence in accordance with a group.  For example I have
the following nomenclature

(block)  (round)   (plot)     (bldg)
  005       050       010       xxx
  005       050       020       xxx
  005       050       030       xxx
  005       050       040       xxx  ...

  005       050       150       xxx  ...

  005       060       010       xxx  ...

  010       010       010       xxx  ...


where xxx represents a unique object identifier.  I'm thinking to use
gen or egen with a cond() qualifier, but how to set the parameters for
the unique object identifier such that it returns sets of the
following structure

  005      050       010         001
  005      050       010         002
  005      050       010         003  ...

  005      050       010         999

thank you,
Ty
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