st: replace missing values in different waves

["benjamin von matt" <b.v.m@gmx.ch>]

hi
 
i have many missing values in wave 2 and wave 3 in the gender variable. there is no missing value in wave 1.
 
how can i replace the missing values in wave 2 and wave 3 with the values of wave 1?
 
best regards
 andrea 
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