"Add checker function(s) to a product.
PRODUCT is a product structure which returned by `product-define'.
The rest arguments CHECKERS should be functions. These functions
-are regist to the product's checkers list, and will be called by
+are registered to the product's checkers list, and will be called by
`product-run-checkers'.
If a checker is `ignore' will be ignored all checkers after this."
(setq product (product-find product))