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This is in the same section as the import feature and is well labeled

Stump Stomp // Burnwillow Clearing maintains that cost and can be cast in either a multicolored, Mono Green, or Mono-Red deck

Whenever a creature you control with a +1/+1 counter on it dies, put a +1/+1 counter on Explorer's Cache

While it did spawn some off the wall "Oops All Spells" decks in formats with a lot of fast mana or Dark Ritual effects, there's no way they could have known this would be possible in a deck with a bunch of lands

Mechanically, you can add one of the options to your spell as you cast it, with stronger effects requiring more mana