/// Return column at `$index` from `$matrix`
/// @group getters
/// @param {Matrix} $matrix - matrix
/// @param {Number} $index - index
/// @return {List}
/// @require {function} get-entry
/// @require {function} rows
/// @require {function} columns
@function get-column($matrix, $index) {
  @if type-of($index) != number {
    @warn "Invalid column index.";
    @return false;
  }

  @if abs($index) > columns($matrix) {
    @warn "Out of bound column index.";
    @return false;
  }

  $column: ();
  @for $i from 1 through rows($matrix) {
    $column: append($column, get-entry($matrix, ($i $index)))
  }

  @return $column;
}