This is the manual for AS, an abstract syntax specification formalism. The main features of this formalism are modularity and support for second-order abstract syntaxes. AS is the first formalism from the CLF computer Languages Factory, a forthcoming set of tools and specification formalisms for quick prototyping and complete implementation of computer languages syntaxes and semantics. This version of AS may be used under the Centaur system for first-order features only. The second-order features will be useful only when a higher-order version of Typol will be distributed.