Add dependency: num-rational.

Cargo.toml

@@ -13,6 +13,8 @@ categories = ["science"]
 
 [dependencies]
 bit-set = "0.4"
+num-rational = { version = "0.1", default-features = false }
+num-traits = "0.1"
 
 [badges]
 travis-ci = { repository = "wangds/puzzle-solver" }

src/lib.rs

@@ -2,6 +2,8 @@
 //! The puzzle rules are expressed as constraints.
 
 extern crate bit_set;
+extern crate num_rational;
+extern crate num_traits;
 
 use std::collections::HashMap;
 use std::ops::Index;