st: announcing mark_changes

[David Kantor <kantor.d@att.net>]

Hello everyone,

Since there was some recent discussion on spells, I decided to put my 
mark_changes program on SSC.

This generates a variable that indicates where values change from one 
observation to the next. It's a simple idea, but the varlist and -by- 
capacity make it very convenient.

Thanks to Kit Baum for placing it in SSC.

--David

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