An interface has the form:
INTERFACE id;
Imports;
Decls
END id.
where id is an identifier that names the interface,
Imports is a sequence of import statements, and Decls is
a sequence of declarations that contains no procedure bodies or
non-constant variable initializations. The names declared in
Decls and the visible imported names must be distinct. It is a static
error for two or more interfaces to form an import cycle.