Keening Stone 
Artifact
{5}, {T}: Target player mills X cards, where X is the number of cards in that player's graveyard.
click a line to search just that ability, click more to combine them
Cards that do something similar 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.
Font of Progress 93%
$0.07#21600
"{3}, {T}: Target player mills X cards, where X is the number of oil counters on this artifact."
Grindstone 86%
#6313
"{3}, {T}: Target player mills two cards. If two cards that share a color were milled this way, repeat this process."
Grindclock 84%
$0.34#13771
"{T}: Target player mills X cards, where X is the number of charge counters on this artifact."