What is the correct way to change the levels of a `factor` column in a `data.table`  (note: not data frame) 

      library(data.table)
      mydt <- data.table(id=1:6, value=as.factor(c("A", "A", "B", "B", "B", "C")), key="id")

      mydt[, levels(value)]
      [1] "A" "B" "C"


I am looking for something like: 

    mydt[, levels(value) <- c("X", "Y", "Z")]

But of course, the above line does not work. 
    
        # Actual               # Expected result
        > mydt                  > mydt
           id value                id value
        1:  1     A             1:  1     X
        2:  2     A             2:  2     X
        3:  3     B             3:  3     Y
        4:  4     B             4:  4     Y
        5:  5     B             5:  5     Y
        6:  6     C             6:  6     Z


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##  THE CORRECT ANSWER: 
        
        # Simply use levels as with any other variable (outside the dt-brakcets[])
        levels(mydt$value) <-  c("X", "Y", "Z")