Resolve syntacic forms like (base ...) and (prefix...)
where the syntax modifies the enclosed sub term.
(FORM DECL... TERM) => where TERM is the result.
(prefix (PREFIXES) TERM) => TERM with prefix names expanded
(base IRI TERM) => TERM with IRIs resolved to absolute IRIs
The DECL part can not itself have nested, independent forms
unless a subclass (carefully) manages that.