An integer vector of row indices. Values must be either all positive
in which case they indicate the rows to select, or all negative in which
case they indicate the rows to omit.
j
An integer vector of column indices. Values must be either all
positive in which case they indicate the columns to select, or all negative
in which case they indicate the columns to omit.
drop
A logical value indicating whether the result should be coerced
to a vector or matrix if possible.