The Prevenience set has three cycles of instants and sorceries ("Chain spells") that resolve an initial effect, then give its caster the option to copy the spell by paying some cost.
The cost to copy the Chain spell for the first cycle doesn't require any mana, but the spell's controller must sacrifice a land, sacrifice a creature, and discard a card. This cycle is Exile, Spawn Simulacra, Epidemic, Rubble, and Cloudburst.
The cost to copy the Chain spell for the second cycle requires only the payment of mana in amount and type equal to the spell's original mana cost. That cycle is Battle Tested, Guided Recollection, Unholy Morass, Reduce-Recycle-Reuse, and Marauding Dilemma.
The cost to copy the Chain spell for the third cycle requires the payment of mana in amount and type equal to the spell's original mana cost and requires discarding one card of a particular color. That cycle is Act of Faith, Homeland Security, Grotesque Result, T.N.T. (The New Threat), and Enlarge.
* The Chain spell's instructions are followed in order.
* Paying to make a Chain spell continue is done as part of the resolution of the spell.
* The copy ability is optional. You choose whether to use it when the original effect resolves.
* There's never more than one "link" in the chain on the stack at once. Each new copy of the Chain spell is created when the previous one resolves. If any of the spells are countered (either by a spell or an ability, or by having an illegal target when it resolves), the chain is broken and no more copies are made.
* For the Chain spells that are copied without paying mana, you can't sacrifice a land or sacrifice a creature or discard a card, unless you can complete all three costs of copying the spell.
* The player putting the copy of the Chain spell on the stack controls that spell.
* The copy of the Chain spell isn't played - it's just put onto the stack - so it won't trigger any ability (such as the ability of Spellseep Moths) that triggers when a spell is played.
* Once the copy of the Chain spell is on the stack, it works just like any other spell. It can be targeted by other spells and abilities that target a spell and it can be countered by spells and abilities that "counter target spell."
* The copy of the Chain spell will have the same targets as the spell it's copying, unless you choose new ones. You may change any number of the targets, and must change the target if required to do so by the language on the card (such as Guided Recollection).
* The copy of the Chain spell has the same text and color as the original spell (including a change in the original spell's color to colorless because of Bajada Moongazer's ability), and can itself being copied.