-PLT/Racket stands to Scheme in something like the relation Firefox stands to HTML. It's one program among others for working with the language; and many of the available programs permit different extensions, have small variations, and so on.
+PLT/Racket stands to Scheme in something like the relation Firefox stands to HTML. It's one program among others for working with the language; and many of those programs (or web browsers) permit different extensions, have small variations, and so on.