acb.dtd   [plain text]

<!ENTITY % a SYSTEM "http://example.org/a/a.dtd">
%a;
<!ENTITY % c SYSTEM "http://example.org/c/c.dtd">
%c;
<!ENTITY % b SYSTEM "http://example.org/b/b.dtd">
%b;
<!ELEMENT acb (a, c, b)>