add sub ops for flat polynomial

src/poly/flat.rs

@@ -1,7 +1,7 @@
 use itertools::Itertools;
 use std::fmt::{self, Display};
 
-use std::ops::{Add, BitXor, Mul};
+use std::ops::{Add, Sub, BitXor, Mul};
 
 use super::var::{StaticVar, Var};
 use std::collections::HashMap;
@@ -75,9 +75,11 @@ impl<V: Var, U: Into<V>> FromIterator<(U, u32)> for Mono<V> {
         term.sort();
 
         // Check duplicate variables
-        assert!((term[..])
-            .windows(2)
-            .all(|window| window[0].0 != window[1].0));
+        assert!(
+            (term[..])
+                .windows(2)
+                .all(|window| window[0].0 != window[1].0)
+        );
 
         Mono { term }
     }
@@ -179,3 +181,16 @@ impl<V: Var> Add for Poly<V> {
         self
     }
 }
+
+impl<V: Var> Sub for Poly<V> {
+    type Output = Poly<V>;
+
+    fn sub(mut self, other: Poly<V>) -> Self::Output {
+        for (mono, coeff) in other.mono {
+            let entry = self.mono.entry(mono).or_insert(0);
+            *entry -= coeff;
+        }
+        self.mono.retain(|_, &mut coeff| coeff != 0);
+        self
+    }
+}