diff --git a/src/field.rs b/src/field.rs
index 0e01e85..706f0af 100644
--- a/src/field.rs
+++ b/src/field.rs
@@ -75,6 +75,16 @@ pub fn mul64(a: &[u64;5], b: &[u64;5]) -> [u64;5] {
     [c0,c1,c2,c3,c4]
 }
 
+fn subtract(a: &[u64; 5], b: &[u64; 5]) -> [u64; 5] {
+    // add p to avoid underflow
+    [ (a[0] + 2251799813685229) - b[0]
+    , (a[1] + 2251799813685247) - b[1]
+    , (a[2] + 2251799813685247) - b[2]
+    , (a[3] + 2251799813685247) - b[3]
+    , (a[4] + 2251799813685247) - b[4]
+    ]
+}
+
 fn from_bytes_64(bytes: &[u8;32]) -> [u64; 5] {
     let low_51_bit_mask = (1u64 << 51) - 1;
     // load bits [  0, 64), no shift