is an all-rates-different model, and \code{"SYM"} is a symmetrical
model. Note that the argument \code{rate} must be of the appropriate
length, i.e., 1, \eqn{k(k - 1)}, or \eqn{k(k - 1)/2} for the three models,
is an all-rates-different model, and \code{"SYM"} is a symmetrical
model. Note that the argument \code{rate} must be of the appropriate
length, i.e., 1, \eqn{k(k - 1)}, or \eqn{k(k - 1)/2} for the three models,