### Lets create some sample data
    set.seed(1)
    LL <- sample(LETTERS[1:5], 20, TRUE)
    DT <- data.table(X=LL, Grp=c("G1", "G2"), key="X")

    DT[X=="B", .I]
    # [1] 1 2 3 4 5 6
    DT[  , .I[X=="B"] ]
    # [1]  1  2  5 11 14 19


## Let's addnother column, and take it a step further
DT[, Grp := c("grpONE", "grpTWO")]

setkey(DT, X)
DT[X=="B", .I]
DT[X=="B"][, .I]
DT[.("B"), .I]$.I

DT[X=="B", .I, by=Grp]$.I


    DT[X=="B", paste(.I, collapse=", ")]
    DT[X=="B", paste(.I, collapse=", "), by=Grp]
    # [1] 1 2 3 4 5 6
    DT[, paste(.I[X=="B"], collapse=", "), by=Grp]