I replaced the rule: aux-pattern with these three patterns:

-------
rule:
   { ?rule-op, ... } => { ?rule-op, ... }
   { ?rule-parser, ...  } => { ?rule-parser, ... }
   { } => { }
rule-op:
   { ?:name(?rule) } => { ?name(?rule) }
rule-parser:
   { ?:name } => { "parse-" ## ?name }
-------

This is slightly better. Now it recurses within the seq(...) calls, but I still get the following error:

-------
Serious warning at tokenizers.dylan:9: Invalid syntax for rule-op in tokenizer-definer macro call. 

tokenizers.dylan:9:                    ---------------
tokenizers.dylan:9:    rule seq(choice(topic-directive, topic-title), opt(topic-content)) => tokens;
tokenizers.dylan:9:                    ---------------

Serious warning at tokenizers.dylan:8: 
  Skipping tokenizer-definer macro call due to previous syntax error. 

tokenizers.dylan:8: ----------------------
tokenizers.dylan:8: define tokenizer topic
tokenizers.dylan:9:    rule seq(choice(topic-directive, topic-title), opt(topic-content)) => tokens;
tokenizers.dylan:11: [...]
tokenizers.dylan:12: end;
tokenizers.dylan:13: ---
-------

It looks like it tries to match "topic-directive" against rule-op, but if that fails, it doesn't try to match it
against rule-parser:?