wp-includes/pomo/translations.php:400Sets translation headers.
$headerstring$valuestring public function set_header( $header, $value ) { parent::set_header( $header, $value ); if ( 'Plural-Forms' === $header ) { list( $nplurals, $expression ) = $this->nplurals_and_expression_from_header( $this->get_header( 'Plural-Forms' ) ); $this->_nplurals = $nplurals; $this->_gettext_select_plural_form = $this->make_plural_form_function( $nplurals, $expression ); } }Introduced in 2.8.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/pomo/translations.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.