/// Checks whether `$matrix` is upper triangular with `$flag` option
/// @group checkers
/// @param {Matrix} $matrix - matrix
/// @param {String | Null} $flag (null) - advanced option, either `null` or `strict` or `unit` or `atom`
/// @return {Bool}
/// @require {function} _is-triangular
@function is-upper-triangular($matrix, $flag: null) {
  @return _is-triangular($matrix, top, $flag);
}