All-Seeing Arbiter 
Creature — Avatar
$0.34 #11279 0.46 4.4 years
Flying
Whenever this creature enters or attacks, draw two cards, then discard a card.
Whenever you discard a card, target creature an opponent controls gets -X/-0 until your next turn, where X is the number of different mana values among cards in your graveyard.
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Cards similar to All-Seeing Arbiter expected a card that isn't here?
Reports become test cases the matcher is graded against before an update ships. Delvefall isn't a chatbot guessing, it compares the printed rules text of every card with plain math, and examples like yours decide which differences should count.
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"Whenever this permanent enters or attacks, draw two cards, then discard a card."
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"Whenever this creature enters or attacks, you may draw a card. If you do, discard a card."
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"Whenever this creature enters or attacks, draw a card, then discard a card."
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$0.26↓#7337↑0.13↓1.6 years↓
"Flying"
Ominous Sphinx82%
$0.15↓#27883↓0.24↓9.1 years↑
"Whenever you cycle or discard a card, target creature an opponent controls gets -2/-0 until end of turn."
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$0.17↓#118390.00↓0.2 years↓
"Flying"
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$2.75↑#6357↑0.421.2 years↓
"Whenever this card enters or attacks, draw a card, then discard a card."
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$0.28↓#8090↑0.00↓9.4 years↑
"Flying"
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$0.13↓#16093↓0.00↓2.2 years↓
"Whenever this creature enters or attacks, it connives."
Syndicate Infiltrator78%
$0.11↓#27274↓0.25↓4.4 years
"Flying"
Tiger Shark, Abyssal Hunter76%
$0.23↓#24569↓0.00↓0.2 years↓
"Whenever this card enters or attacks, he connives."
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$0.22↓#21243↓0.57↑6.9 years↑
"Whenever this creature enters or attacks, you may discard a card. If you do, draw a card."
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$0.27↓#95950.06↓3.0 years↓
"Flying"
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$0.21↓#19362↓0.00↓6.2 years↑
"When this Aura enters, draw two cards, then discard a card."
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$0.90↑#97790.06↓2.8 years↓
"Flying"
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$0.15↓#23930↓0.12↓4.9 years
"Whenever this creature attacks, draw a card, then discard a card."
+1 more matching line
Adventurer's Airship74%
$0.19↓#108210.22↓1.2 years↓
"Whenever this Vehicle attacks, draw a card, then discard a card."
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"Flying"
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$0.55↑#119370.27↓4.8 years
"Flying"