/// Swaps entries `$e1` and `$e2` from `$matrix`
/// @group swap
/// @param {Matrix} $matrix - matrix
/// @param {Number} $e1 - (x y) coordinates of first entry
/// @param {Number} $e2 - (x y) coordinates of second entry
/// @return {Matrix}
/// @require {function} _valid-coords
/// @require {function} get-entry
/// @require {function} set-entry
@function swap-entries($matrix, $e1, $e2) {
  @if _valid-coords($e1, $matrix) and _valid-coords($e2, $matrix) {
    $tmp: get-entry($matrix, $e1);
    $matrix: set-entry(set-entry($matrix, $e1, get-entry($matrix, $e2)), $e2, $tmp);
    @return $matrix;
  }
  @return false;
}