A Camlp4 preprocessor operates by loading a collection of compiled modules which define a parser as well as a pretty-printer: the parser converts an input program into an internal representation. This internal representation constitutes the abstract syntax tree (AST). It can be output in a binary form, e.g. it can be passed directly to one of the OCaml compilers, or it can be converted back into a clear text program. The notion of "concrete syntax" refers to the format in which the "abstract syntax" is represented.