Here is the replacement for the original rule: aux pattern

----
rule:
   { seq(?nested-rule) ... } => { seq(?nested-rule) ... }
   { choice(?nested-rule) ... } => { choice(?nested-rule) ... }
   { many(?nested-rule) ... } => { many(?nested-rule) ... }
   { opt(?nested-rule) ... } => { opt(?nested-rule) ... }
   { opt-seq(?nested-rule) ... } => { opt-seq(?nested-rule) ... }
   { opt-choice(?nested-rule) ... } => { opt-choice(?nested-rule) ... }
   { opt-many(?nested-rule) ... } => { opt-many(?nested-rule) ... }
   { req-next(?nested-rule) ... } => { req-next(?nested-rule) ... }
   { not-next(?nested-rule) ... } => { not-next(?nested-rule) ... }
   { ?:name ... } => { "parse-" ## ?name ... }
   { ?:token ... } => { ?token ... }
   { } => { }
nested-rule:
   { ?rule } => { ?rule }
----

Aux patterns must match everything, and recursion into an expression can be done via nested-rule as I show here.