The general format of EDIF involves using parentheses to delimit data definitions, and in this way it superficially resembles Lisp. The basic tokens of EDIF 2.0.0 were keywords (like library, cell, instance, etc.), strings (delimited with double quotes), integer numbers, symbolic constants (e.g. GENERIC, TIE, RIPPER for cell types) and "Identifiers", which are reference labels formed from a very restricted set of characters. EDIF 3.0.0 and 4.0.0 dropped the symbolic constants entirely, using keywords instead.