

   LLoowweerr aanndd UUppppeerr TTrriiaanngguullaarr PPaarrtt ooff aa MMaattrriixx

        lower.tri(x, diag=FALSE)
        upper.tri(x, diag=FALSE)

   AArrgguummeennttss::

          x: a matrix.

       diag: logical. Should the diagonal be included?

   VVaalluuee::

        Returns a `logical' matrix of the same dimension as `x'
        which is `TRUE' for the relevant triangular part of
        `x'.

   SSeeee AAllssoo::

        `diag', `matrix'.

   EExxaammpplleess::

        m2 <- ma <- matrix(1:20, 4, 5)
        m2[lower.tri(m2)] <- NA
        m2
        all(lower.tri(ma) == !upper.tri(ma, diag=TRUE))

