This page contains the reasoning behind the KirriCorp League's errata on the card Corile, shown below in its original form.

Name: Corile
Type: Creature
Race: Cyber Lord
Civilization: Water
Cost: 5
Rules Text: When you put this creature into the battle zone, choose one of your opponent's creatures in the battle zone and put it on top of his deck.
Power: 2000
Mana Number: 1
Set: Evo-Crushinators of Doom

As with all KC errata, justification of this argument relies on the semi-rigid card costing system given within Duel Masters. The relevant aspects of the cost system are shown, with examples, in the following data. It is assumed that all cards should have equal cost-to-power ratios until cost 5. After this, they gain twice as much power for every point of cost added.

Ability: To attack a player (either breaking a shield or delivering a final blow)
Cost: The card itself, once it is summoned.

Ability: Unconditional removal of a creature from the battle zone back to the hand.
Cost: -3000 power when it is the 'battle zone entry' ability of a creature.
Reference: Unicorn Fish, Aqua Surfer(-1000 power for Shield Trigger), King Mazelan, Aqua Sniper (+1000 power Super Rare Bonus)

Ability: 'Opponent's choice' removal of a creature from the battle zone to the graveyard.
Cost: -7000 power when it is the 'battle zone entry' ability of a creature
Reference: Swamp Worm
Note: This ability is from the Base Set and as such may be 'overcosted'.

Ability: 'Opponent's choice' removal of a creature from the battle zone to the mana zone.
Cost: -7000 power when it is the 'battle zone entry' ability of a creature
Reference: Storm Shell
Note: This ability is from the Base Set and as such may be 'overcosted'.

Ability: Conditional removal (2000 power or less) of a creature from the battle zone to the graveyard.
Cost: -3000 power when it is the 'battle zone entry' ability of a creature.
Reference: Meteosaur

Two things of note are important here. Firstly, that Corile allows the choice regarding the removal of the creature, which is not odd for the line of cards that it comes from. Secondly, that other cards which remove a creature from the field create a resource elsewhere.

Any creature sent to the mana zone becomes a resource, though its usefulness is diminished. It can be regained through varying effects, put to use, or may affect the abilities of certain cards. Any creature sent to the graveyard is also a resource, with further diminished usefulness. However, destruction triggers many effects, and as such, the destruction of certain cards is in itself a resource.

A creature sent to the top of the deck ceases to be a resource, though it may seem to be one by the reasoning of draw effects.

Consider Corile vs Swamp Worm, with Hypersquid Walter and Amber Piercer, respectively, available to the opponent.

Both cards remove one opposing creature from the battle zone. Both cards may attack on the next turn if they are not removed.

Corile can be played for two mana less than a Swamp Worm. If we assume that the worm is overcosted and hypothetically reduce its cost to five also, we move onto the next point.

Corile allows the player doing the summoning to choose what is removed. The Swamp Worm does not. If we again equate the Worm for 'fairness', we move on to the next point.

Corile removes access to the player's next card. It can be argued that if Corile is used on any of the dozen or so creatures that search for other creatures or draw cards as their 'battle zone entry' effect, that the next card is accessible in that way. However, it should be noted that if Corile returned the card only to hand, then two cards would be available. The argument that 'if Corile did not remove the card then it would be the same thing' is also not true because the creature, having been previously summoned, would not have summoning sickness.

There is no fundamental difference between having a Swamp Worm destroy Magris, then drawing and summoning a different Magris... and having one Magris returned via Corile's effect. This means that a creature must draw a new card when summoned in order to be 'equally' affected by Corile and a Swamp Worm, even if we change the Worm to cost five and give the summoning player the choice of target.

Corile does not trigger the 'destruction' abilities of cards. Swamp Worm will always trigger these abilities. I.e. it is safe to remove Schuka, Duke of Amnesia, with Corile, but not as safe to do so with a Swamp Worm. In the case of the Storm Shell, the fact that the ability is also not triggered is balanced by the fact that the opponent has gained a new resource in mana, which is a more accessible and useful resource for most decks than a new resource in grave.

Now, finally, we move into the point of the other creature on the opposing side. Following a Corile summon, Hypersquid Walter may attack, returning the creature to the hand. Following a Swamp Worm summon, Amber Piercer may attack, returning the creature to the hand.

Two points again need to be considered. Firstly, that Corile removes the choice aspect inherent in the Amber Piercer/Swamp Worm scenario. The player is not choosing what comes from the deck in the way that they would be choosing what to bring back from the grave. They already know exactly what they are getting. It was chosen by the Corile player.

Secondly, if one reverses the situation, such that Corile is facing the Amber Piercer and Hypersquid Walter facing the Swamp Worm, the Piercer has no capacity to return the creature to the hand. Walter does not either. Walter will bring a new card when it attacks, and the creature destroyed by the Swamp Worm will remain in the grave. The Piercer will revive any fallen creature, and the card returned by Corile will be drawn as next card.

This is an almost even scenario, with only an advantage of 'more cards available overall' going to Walter. This is the definition of Corile's effect. It is the only creature that can reduce the overall number of cards available to a player unconditionally. It will do this, by one, every time it is summoned and affects a normal creature, and will do more reducing if it targets an evolution creature.

It is for this specific reason that Corile was errataed, and its errata is meant solely to correct this ability:
"Reduce the number of cards available to your opponent by at least one."

Therefore the applied errata has left the unconditional removal aspect intact, the player's ability to choose the target intact, and Corile's original cost-power ratio intact. It has removed only the ability to reduce the total number of cards available. As 'compensation', Corile's effect has been extended to give the player the choice of targeting their own cards, and of choosing no target at all should it suit them.

Result:

Name: Corile
Type: Creature
Race: Cyber Lord
Civilization: Water
Cost: 5
Rules Text: When you put this creature into the battle zone, you may choose a creature in the battle zone and return it to it's owner's hand.
Power: 2000
Mana Number: 1
Set: Evo-Crushinators of Doom