A vocabulary (or signature) is a finite collection of
function names, each of a fixed arity. Some function names may be marked
as relation names or static names, or both. Every
vocabulary contains the following static names: the equality sign, nullary
function names true, false, undef and the names of the usual
Boolean operations. The equality sign and true, false are marked as
relation names. The Greek letter is reserved to denote
vocabularies.