Library of Leng + Madness

[No_Life_No_Future]

502.24 Madness
502.24a Madness is a keyword that represents two abilities. The first is a static ability that functions while the card with madness is in a player's hand. The second is a triggered ability that functions when the first ability is applied. "Madness [cost]" means "If a player would discard this card, that player discards it, but may remove it from the game instead of putting it into his or her graveyard" and "When this card is removed from the game this way, its owner may play it by paying [cost] rather than paying its mana cost. If that player doesn't, he or she puts this card into his or her graveyard."
502.24b Playing a spell using its madness ability follows the rules for paying alternative costs in rules 409.1b and 409.1f-h.

Library of Leng
1, Artifact
You have no maximum hand size.
If an effect causes you to discard a card, discard it, but you may put it on top of your library instead of into your graveyard.

I am guessing that if you chose the order in which the replacement abilities go on the stack. If you use LoL (hehe) to put the card with madness on top then you cant RFG it and cast the card. And if you RFG the card and cast it you would not be able to use the on top of library replacement effect.

Am I correct on this interaction?

[Finn]

Replacement abilities do not get applied via the stack. Only one of them can be applied to any action as it "replaces" the action. So the answer is you are correct in the outcome atleast, you can't do both.

[cdr]

419.9a If two or more replacement 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. Then the other effect applies if it is still appropriate. If one or more of the applicable replacement effects is a self-replacement effect (see rule 419.6d), that effect is applied before any other replacement effects. If two or more players have to make these choices at the same time, choices are made in APNAP order (see rule 103.4).
Example: Two permanents are in play. One is an enchantment that reads "If a card would be put into a graveyard, instead remove it from the game," and the other is a creature that reads "If [this creature] would be put into a graveyard from play, instead shuffle it into its owner's library." The controller of the creature that would be destroyed decides which replacement to apply first; the other does nothing.