diff --git a/py/mpz.c b/py/mpz.c
index 9e60fc5..9c42878 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -218,6 +218,38 @@
     return idig + 1 - oidig;
 }
 
+/* computes i = j & -k = j & (~k + 1)
+   returns number of digits in i
+   assumes enough memory in i; assumes normalised j, k
+   can have i, j, k pointing to same memory
+*/
+STATIC uint mpn_and_neg(mpz_dig_t *idig, const mpz_dig_t *jdig, uint jlen, const mpz_dig_t *kdig, uint klen) {
+    mpz_dig_t *oidig = idig;
+    mpz_dbl_dig_t carry = 1;
+
+    for (; jlen > 0 && klen > 0; --jlen, --klen, ++idig, ++jdig, ++kdig) {
+        carry += *kdig ^ DIG_MASK;
+        *idig = (*jdig & carry) & DIG_MASK;
+        carry >>= DIG_SIZE;
+    }
+
+    for (; jlen > 0; --jlen, ++idig, ++jdig) {
+        carry += DIG_MASK;
+        *idig = (*jdig & carry) & DIG_MASK;
+        carry >>= DIG_SIZE;
+    }
+
+    if (carry != 0) {
+        *idig = carry;
+    } else {
+        // remove trailing zeros
+        for (--idig; idig >= oidig && *idig == 0; --idig) {
+        }
+    }
+
+    return idig + 1 - oidig;
+}
+
 /* computes i = j | k
    returns number of digits in i
    assumes enough memory in i; assumes normalised j, k; assumes jlen >= klen
@@ -896,24 +928,35 @@
    can have dest, lhs, rhs the same
 */
 void mpz_and_inpl(mpz_t *dest, const mpz_t *lhs, const mpz_t *rhs) {
-    // make sure lhs has the most digits
-    if (lhs->len < rhs->len) {
-        const mpz_t *temp = lhs;
-        lhs = rhs;
-        rhs = temp;
-    }
-
     if (lhs->neg == rhs->neg) {
-        mpz_need_dig(dest, rhs->len);
-        dest->len = mpn_and(dest->dig, lhs->dig, lhs->len, rhs->dig, rhs->len);
+        if (lhs->neg == 0) {
+            // make sure lhs has the most digits
+            if (lhs->len < rhs->len) {
+                const mpz_t *temp = lhs;
+                lhs = rhs;
+                rhs = temp;
+            }
+            // do the and'ing
+            mpz_need_dig(dest, rhs->len);
+            dest->len = mpn_and(dest->dig, lhs->dig, lhs->len, rhs->dig, rhs->len);
+            dest->neg = 0;
+        } else {
+            // TODO both args are negative
+            assert(0);
+        }
     } else {
-        mpz_need_dig(dest, lhs->len);
-        // TODO
-        assert(0);
-//        dest->len = mpn_and_neg(dest->dig, lhs->dig, lhs->len, rhs->dig, rhs->len);
+        // args have different sign
+        // make sure lhs is the positive arg
+        if (rhs->neg == 0) {
+            const mpz_t *temp = lhs;
+            lhs = rhs;
+            rhs = temp;
+        }
+        mpz_need_dig(dest, lhs->len + 1);
+        dest->len = mpn_and_neg(dest->dig, lhs->dig, lhs->len, rhs->dig, rhs->len);
+        assert(dest->len <= dest->alloc);
+        dest->neg = 0;
     }
-
-    dest->neg = lhs->neg;
 }
 
 /* computes dest = lhs | rhs