diff --git a/src/card.rs b/src/card.rs
index dd2fa5e..7be1603 100644
--- a/src/card.rs
+++ b/src/card.rs
@@ -5,7 +5,7 @@ use rand::distr::{Distribution, StandardUniform};
 use rand::Rng;
 use std::fmt::{Display, Formatter};
 
-#[derive(Copy, Clone)]
+#[derive(Copy, Clone, PartialEq)]
 pub struct Card {
     v: Vector4<F3>,
 }
@@ -21,6 +21,11 @@ impl Card {
             ),
         }
     }
+
+    pub fn is_set(a: Self, b: Self, c: Self) -> bool {
+        let (a, b, c) = (a.v, b.v, c.v);
+        a + b + c == Vector4::<F3>::zero()
+    }
 }
 
 impl Display for Card {
@@ -72,6 +77,19 @@ impl CardSet {
         }
     }
 
+    pub fn contains_set(&self) -> bool {
+        for i in 0..(self.cards.len() - 2) {
+            for j in (i + 1)..(self.cards.len() - 1) {
+                for k in (j + 1)..(self.cards.len()) {
+                    if Card::is_set(self.cards[i], self.cards[j], self.cards[k]) {
+                        return true;
+                    }
+                }
+            }
+        }
+        false
+    }
+
     fn grid(&self) -> [[[[bool; 3]; 3]; 3]; 3] {
         let mut grid = [[[[false; 3]; 3]; 3]; 3];
         for card in &self.cards {
@@ -205,4 +223,55 @@ mod tests {
             }
         }
     }
+
+    #[test]
+    fn set_stays_set() {
+        let mut rng = rng();
+        let mut cards = CardSet::new(&[
+            Card::new(0, 0, 0, 0),
+            Card::new(1, 1, 1, 1),
+            Card::new(2, 2, 2, 2),
+        ]);
+        assert!(cards.contains_set());
+
+        for _ in 0..1000 {
+            let mapping: CardMapping = rng.sample(StandardUniform);
+            cards = mapping.map(cards);
+            assert!(cards.contains_set());
+        }
+    }
+
+    #[test]
+    fn noset_stays_noset() {
+        let mut rng = rng();
+        let mut cards = CardSet::new(&[
+            Card::new(0, 0, 0, 1),
+            Card::new(0, 0, 1, 0),
+            Card::new(0, 0, 1, 2),
+            Card::new(0, 0, 2, 1),
+            Card::new(0, 1, 1, 1),
+            Card::new(0, 2, 0, 0),
+            Card::new(0, 2, 0, 2),
+            Card::new(0, 2, 2, 0),
+            Card::new(0, 2, 2, 2),
+            Card::new(1, 0, 1, 1),
+            Card::new(1, 2, 1, 1),
+            Card::new(2, 0, 0, 0),
+            Card::new(2, 0, 0, 2),
+            Card::new(2, 0, 2, 0),
+            Card::new(2, 0, 2, 2),
+            Card::new(2, 1, 1, 1),
+            Card::new(2, 2, 0, 1),
+            Card::new(2, 2, 1, 0),
+            Card::new(2, 2, 1, 2),
+            Card::new(2, 2, 2, 1),
+        ]);
+        assert!(!cards.contains_set());
+
+        for _ in 0..1000 {
+            let mapping: CardMapping = rng.sample(StandardUniform);
+            cards = mapping.map(cards);
+            assert!(!cards.contains_set());
+        }
+    }
 }