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add_filter( 'comment_form_defaults', 'nabm_change_comment_label' );
 
function nabm_change_comment_label( $args ) {
	$args['comment_field'] = '<p class="comment-form-comment"><label for="comment">' . _x( 'Your Comment', 'noun' ) . '</label> <textarea id="comment" name="comment" cols="45" rows="8" aria-required="true"></textarea></p>';
	return $args;
}