Hero Limiting

how can I manually manage (with triggers) the hero limit (1 hero with a lvl 1 towhall, 2 heroes with a lvl 2 townhall and so on)

by now I'm using this trigger:
Event

• Unit_ - A unit ends a unit training

Condition

• ((Unit-type of (Trained unit)) is A Hero) Equals TRUE

Action

• Player - Limit training of (Unit-type of (Trained unit)) to 0 for (Owner of (Trained unit))

but this just limits the production of the heroes to one per type

suggestions are welcome

EDIT: the code to replace lists doesn't work really well with indented lists