Grindstone 
Artifact
{3}, {T}: Target player mills two cards. If two cards that share a color were milled this way, repeat this process.
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.
Keening Stone 86%
$0.42#9175
"{5}, {T}: Target player mills X cards, where X is the number of cards in that player's graveyard."
Font of Progress 85%
$0.07#21600
"{3}, {T}: Target player mills X cards, where X is the number of oil counters on this artifact."