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true: The Immortal Sun reduces all my spell costs.

However, it cannot reduce certain creature spell costs.
(Alternative costs available or capable of cost reduction in its own right?)

e.g.
-Hooting Mandrills
-Errant Ephemeron

These kind of interactions between alternate costs, cost mods, etc. have never been very good. It's the next thing to do on my to do list once I finish triggers.