st: identify unique string values within lists of elements over chosen time windows

[Denisa Mindruta <mdenisa@yahoo.com>]

Hi everyone. I have an unbalanced, large panel dataset, where each observation 
can take multiple string values (each string is stored in a separate variable). 
At each point in time, I need to count whether the string value(s) taken by an 
observation are "new" , meaning that they do not show up among the values taken 
by the same observation in previous years. How should I approach  this problem ? 
Thanks !  Below is a description of data. I need to calculate newval

obs     yr   var1    var2    var3    newval
1        90   str1    str2    str3     3
1        91    str1    str4    str5     2
2        90    str3    str4              2
2        91    str4    str5              1
2        93    str3    str5              0
2        94    str7                       1  

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