+
+Rules can apply not only to L<LaTeX::TikZ::Set> consumer objects, but also to the L<LaTeX::TikZ::Mod> consumer objects they contain.
+The are stored as L<LaTeX::TikZ::Functor::Rule> objects.
+
+When the functor is called onto a set object, all its associated rules are tried successively, and the handler of the first matching rule is executed with :
+
+=over 4
+
+=item *
+
+the functor object as its first argument ;
+
+=item *
+
+the current set object as its second argument ;
+
+=item *
+
+the arguments passed to the functor itself starting at the third argument.
+
+=back
+
+The handler is expected to return the new set/mod that will in the resulting set tree.
+If the new set is different from the original, then the functor is applied to all the mods of the set, and their cloned version are added to the new set.
+
+If no matching rule is found, the object is returned as-is.