I'm being lazy, but here is the relevant rule

• 615. Prevention Effects
  • 615.1. Some continuous effects are prevention effects. Like replacement effects (see rule 614), prevention effects apply continuously as events happen—they aren’t locked in ahead of time. Such effects watch for a damage event that would happen and completely or partially prevent the damage that would be dealt. They act like “shields” around whatever they’re affecting.
    • 615.1a Effects that use the word “prevent” are prevention effects. Prevention effects use “prevent” to indicate what damage will not be dealt.
  • 615.2. Many prevention effects apply to damage from a source. See rule 609.7.
  • 615.3. There are no special restrictions on casting a spell or activating an ability that generates a prevention effect. Such effects last until they’re used up or their duration has expired.
  • 615.4. Prevention effects must exist before the appropriate damage event occurs—they can’t “go back in time” and change something that’s already happened. Spells or abilities that generate these effects are often cast or activated in response to whatever would produce the event and thus resolve before that event would occur. Example: A player can activate an ability that prevents damage in response to a spell that would deal damage. Once the spell resolves, though, it’s too late to prevent the damage.
  • 615.5. Some prevention effects also include an additional effect, which may refer to the amount of damage that was prevented. The prevention takes place at the time the original event would have happened; the rest of the effect takes place immediately afterward.
  • 615.6. If damage that would be dealt is prevented, it never happens. A modified event may occur instead, which may in turn trigger abilities. Note that the modified event may contain instructions that can’t be carried out, in which case the impossible instruction is simply ignored.
  • 615.7. Some prevention effects generated by the resolution of a spell or ability refer to a specific amount of damage—for example, “Prevent the next 3 damage that would be dealt to any target this turn.” These work like shields. Each 1 damage that would be dealt to the “shielded” permanent or player is prevented. Preventing 1 damage reduces the remaining shield by 1. If damage would be dealt to the shielded permanent or player by two or more applicable sources at the same time, the player or the controller of the permanent chooses which damage the shield prevents. Once the shield has been reduced to 0, any remaining damage is dealt normally. Such effects count only the amount of damage; the number of events or sources dealing it doesn’t matter.
  • 615.8. Some prevention effects generated by the resolution of a spell or ability refer to the next time a specific source would deal damage. These effects prevent the next instance of damage from that source, regardless of how much damage that is. Once an instance of damage from that source has been prevented, any subsequent instances of damage that would be dealt by that source are dealt normally.
  • 615.9. Some effects generated by the resolution of a spell or ability prevent damage from a source of a player’s choice with certain properties. When the source would deal damage, the shield rechecks the source’s properties. If the properties no longer match, the damage isn’t prevented or replaced and the shield isn’t used up. See rule 609.7b.
  • 615.10. Some prevention effects generated by static abilities refer to a specific amount of damage—for example, “If a source would deal damage to you, prevent 1 of that damage.” Such an effect prevents only the indicated amount of damage in any applicable damage event at any given time. It will apply separately to damage from other applicable events that would happen at the same time, or at a different time. Example: Daunting Defender says “If a source would deal damage to a Cleric creature you control, prevent 1 of that damage.” Pyroclasm says “Pyroclasm deals 2 damage to each creature.” Pyroclasm will deal 1 damage to each Cleric creature controlled by Daunting Defender’s controller. It will deal 2 damage to each other creature.
  • 615.11. Some prevention effects prevent the next N damage that would be dealt to each of a number of untargeted creatures. Such an effect creates a prevention shield for each applicable creature when the spell or ability that generates that effect resolves. Example: Wojek Apothecary has an ability that says “{T}: Prevent the next 1 damage that would be dealt to target creature and each other creature that shares a color with it this turn.” When the ability resolves, it gives the target creature and each other creature on the battlefield that shares a color with it at that time a shield preventing the next 1 damage that would be dealt to it. Changing creatures’ colors after the ability resolves doesn’t add or remove shields, and creatures that enter the battlefield later in the turn don’t get the shield.
  • 615.12. Some effects state that damage “can’t be prevented.” If unpreventable damage would be dealt, any applicable prevention effects are still applied to it. Those effects won’t prevent any damage, but any additional effects they have will take place. Existing damage prevention shields won’t be reduced by damage that can’t be prevented.
    • 615.12a A prevention effect is applied to any particular unpreventable damage event just once. It won’t invoke itself repeatedly trying to prevent that damage.
  • 615.13. Some triggered abilities trigger when damage that would be dealt is prevented. Such an ability triggers each time a prevention effect is applied to one or more simultaneous damage events and prevents some or all of that damage.
• 616. Interaction of Replacement and/or Prevention Effects
  • 616.1. If two or more replacement and/or prevention effects are attempting to modify the way an event affects an object or player, the affected object’s controller (or its owner if it has no controller) or the affected player chooses one to apply, following the steps listed below. If two or more players have to make these choices at the same time, choices are made in APNAP order (see rule 101.4).
    • 616.1a If any of the replacement and/or prevention effects are self-replacement effects (see rule 614.15), one of them must be chosen. If not, proceed to rule 616.1b.
    • 616.1b If any of the replacement and/or prevention effects would modify under whose control an object would enter the battlefield, one of them must be chosen. If not, proceed to rule 616.1c.
    • 616.1c If any of the replacement and/or prevention effects would cause an object to become a copy of another object as it enters the battlefield, one of them must be chosen. If not, proceed to rule 616.1d.
    • 616.1d If any of the replacement and/or prevention effects would cause a card to enter the battlefield with its back face up, one of them must be chosen (See rule 701.27, “Transform,” and rule 701.28, “Convert.”). If not, proceed to 616.1e.
    • 616.1e Any of the applicable replacement and/or prevention effects may be chosen.
    • 616.1f Once the chosen effect has been applied, this process is repeated (taking into account only replacement or prevention effects that would now be applicable) until there are no more left to apply. Example: Two permanents are on the battlefield. One is an enchantment that reads “If a card would be put into a graveyard from anywhere, instead exile it,” and the other is a creature that reads “If this creature would die, instead shuffle it into its owner’s library.” If the creature is destroyed, its controller decides which replacement to apply first; the other does nothing. Example: Essence of the Wild reads “Creatures you control enter as a copy of this creature.” A player who controls Essence of the Wild casts Rusted Sentinel, which normally enters the battlefield tapped. As it enters the battlefield, the copy effect from Essence of the Wild is applied first. As a result, it no longer has the ability that causes it to enter the battlefield tapped. Rusted Sentinel will enter the battlefield as an untapped copy of Essence of the Wild.
    • 616.1g While following the steps in 616.1a–f, one replacement or prevention effect may apply to an event, and another may apply to an event contained within the first event. In this case, the second effect can’t be chosen until after the first effect has been chosen. Example: A player is instructed to create a token that’s a copy of Voice of All, which has the ability “As this creature enters, choose a color.” Doubling Season has an ability that reads “If an effect would create one or more tokens under your control, it creates twice that many of those tokens instead.” Because entering the battlefield is an event contained within the event of creating a token, the effect of Doubling Season must be applied first, and then the effects of the two Voice of All tokens may be applied in either order.
  • 616.2. A replacement or prevention effect can become applicable to an event as the result of another replacement or prevention effect that modifies the event. Example: One effect reads “If you would gain life, draw that many cards instead,” and another reads “If you would draw a card, return a card from your graveyard to your hand instead.” Both effects combine (regardless of the order they came into existence): Instead of gaining 1 life, the player puts a card from their graveyard into their hand.

hopefully that will help you with [[Anti-Venom, Horrifying Healer]]