1 files changed, 16 insertions, 1 deletions
diff --git a/aoc-2020-gleam/src/ext/setx.gleam b/aoc-2020-gleam/src/ext/setx.gleam
index 4a9c0c1..f2c67e8 100644
--- a/aoc-2020-gleam/src/ext/setx.gleam
+++ b/aoc-2020-gleam/src/ext/setx.gleam
@@ -1,6 +1,7 @@
 import gleam/list
 import gleam/set.{type Set}
 import gleam/iterator as iter
+import ext/resultx as resx
 import ext/iteratorx as iterx
 
 pub fn count(s: Set(a), satisfying predicate: fn(a) -> Bool) -> Int {
@@ -17,11 +18,21 @@ pub fn map(s: Set(a), with fun: fn(a) -> b) -> Set(b) {
   |> set.from_list
 }
 
+pub fn arbitrary_union(of sets: List(Set(a))) -> Set(a) {
+  list.fold(over: sets, from: set.new(), with: set.union)
+}
+
+pub fn arbitrary_intersection(of sets: List(Set(a))) -> Set(a) {
+  sets
+  |> list.reduce(with: set.intersection)
+  |> resx.assert_unwrap
+}
+
 pub fn flat_map(s: Set(a), with fun: fn(a) -> Set(b)) -> Set(b) {
   s
   |> set.to_list
   |> list.map(with: fun)
-  |> list.fold(from: set.new(), with: set.union)
+  |> arbitrary_union
 }
 
 pub fn toggle(in s: Set(a), this value: a) -> Set(a) {
@@ -31,3 +42,7 @@ pub fn toggle(in s: Set(a), this value: a) -> Set(a) {
     False -> set.insert(into: _, this: value)
   }
 }
+
+pub fn subtract(from a: Set(a), given b: Set(a)) -> Set(a) {
+  set.drop(a, set.to_list(b))
+}