An abstract syntax is a recursive definition of a formalism, some operators (constructors), and some types (also called phyla) designed by their names. These names are identifiers. A name can be used before it has been define.
There is no rule on the appearance of the identifiers for operators and phyla. One can chose to begin phyla names by an upper case letter, an operator names by a lower case as it is done in most of the examples in this manual, but this rule is not part of the language.
The name ``meta'' is not allowed for an operator as it has a special meaning in the Centaur VTP. When creating a formalism, a special operator named ``meta'' used to create and manipulate schemes (or incomplete trees) in Centaur is added to every phyla of the syntax.
It is possible to give the same name for an operator and a phylum or a formalism. One can think that this possibility is useful when a phylum contains exactly one operator, for example. However, the AS type-checker will emit a warning when a name is overloaded. Double definitions of an operator or of a phylum are not allowed and result in error messages.
It is not possible to give the same name for a formalism and a phylum of this formalism as the formalism may be viewed as the type union of all the phyla belonging to the formalism.
Thierry Despeyroux