

   WWhhiicchh iinnddiicceess aarree TTRRUUEE ??

        which(x)

   AArrgguummeennttss::

          x: a logical vector or array.

   VVaalluuee::

        An integer vector with `length' equal to `sum(x)', i.e.
        to the number of `TRUE's in `x'; Basically, the result
        is `(1:length(x))[x]'.

   SSeeee AAllssoo::

        `Logic'

   EExxaammpplleess::

        which(LETTERS == "R")
        which((1:12)%%2 == 0) #which are even?

