Sprintf final class

A pattern filled one fragment at a time, as wp_sprintf fills it: "%%" is a percent sign, each other fragment (a directive and the text up to the next one) takes the next argument or the one its position names, and $filter may write a fragment itself (the wp_sprintf filter: the list directive %l lives there); otherwise PHP's sprintf fills it, an array as nothing, and a fragment it cannot fill stays as written.