15.3.1 Lexical contours and rib objects

A rib is an object constructed at every lexical contour in the source code to hold informations about the syntactic bindings introduced in that contour; “lexical contours” are, for example, let and similar syntaxes that can introduce bindings.

The purpose of rib objects is to map syntactic binding’s source names (the symbols as they appear in the source code) to the associated label gensyms; this map is used when establishing if a syntax object identifier in reference position is captured by an existing syntactic binding.