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-rw-r--r--aoc2021/aoc2021.csproj1
1 files changed, 1 insertions, 0 deletions
diff --git a/aoc2021/aoc2021.csproj b/aoc2021/aoc2021.csproj
index 7180a09..b554430 100644
--- a/aoc2021/aoc2021.csproj
+++ b/aoc2021/aoc2021.csproj
@@ -15,6 +15,7 @@
 
   <ItemGroup>
     <Using Include="System.Collections.Generic" />
+    <Using Include="System.Collections.Immutable" />
     <Using Include="System.Diagnostics" />
     <Using Include="System.Reflection" />
     <Using Include="System.Text" />