Add puzzle type.

src/lib.rs

@@ -2,3 +2,7 @@
 //! The puzzle rules are expressed as constraints.
 
 extern crate bit_set;
+
+pub use puzzle::Puzzle;
+
+mod puzzle;

src/puzzle.rs

@@ -0,0 +1,23 @@
+//! The puzzle's state and rules.
+
+/// The puzzle to be solved.
+#[allow(dead_code)]
+pub struct Puzzle {
+    // The number of variables in the puzzle.
+    num_vars: usize,
+}
+
+impl Puzzle {
+    /// Allocate a new puzzle.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// puzzle_solver::Puzzle::new();
+    /// ```
+    pub fn new() -> Self {
+        Puzzle {
+            num_vars: 0,
+        }
+    }
+}