diff --git a/Cargo.toml b/Cargo.toml
index 18f6d2d..c286ffd 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -5,3 +5,4 @@ authors = ["David Wang <millimillenary@gmail.com>"]
 license = "MIT"
 
 [dependencies]
+bit-set = "*"
diff --git a/src/lib.rs b/src/lib.rs
index 3f1cc4b..e49bfe5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,2 +1,4 @@
 //! This crate searches for the solutions to logic puzzles.
 //! The puzzle rules are expressed as constraints.
+
+extern crate bit_set;