LOCF

input a 
1
2
.
3
.
4
end

**suppose I want to replace each missing observation by the observation preceding it

replace a = a[_n-1] if missing(a)
. l

     +---+
     | a |
     |---|
  1. | 1 |
  2. | 2 |
  3. | 2 |
  4. | 3 |
  5. | 3 |
     |---|

  6. | 4 |