diff --git a/bigjoyints/big.py b/bigjoyints/big.py
index e0aedc0..ddb5e4b 100644
--- a/bigjoyints/big.py
+++ b/bigjoyints/big.py
@@ -1,3 +1,4 @@
+from copy import copy
 from itertools import zip_longest
 from pprint import pprint as P
 import  unittest
@@ -22,10 +23,10 @@ class BigInt:
     def digitize(n):
         if n < 0:
             raise ValueError(f'Non-negative only: {n}')
-        if not n:
-            yield OberonInt(0)
-            return  # Not strictly needed as the following while
-                    # will not do anything for n == 0.
+        #if not n:
+        #    yield OberonInt(0)
+        #    return  # Not strictly needed as the following while
+        #            # will not do anything for n == 0.
         while n:
             n, digit = divmod(n, 2**31)
             yield OberonInt(digit)
@@ -43,12 +44,33 @@ class BigInt:
             n = -n
         return n
 
+    def negate(self):
+        result = BigInt()
+        result.sign = not self.sign
+        result.digits = [
+            OberonInt(digit.value ^ 2**31 - 1)
+            for digit in self.digits
+            ]
+        return result
+
     def __add__(self, other):
         if not isinstance(other, BigInt):
             other = BigInt(other)
         if self.sign == other.sign:
             return self.add_like_signs(other)
 
+        return self.add_unlike_signs(other)
+
+    def __sub__(self, other):
+        if not isinstance(other, BigInt):
+            other = BigInt(other)
+        #print(23)
+        #print(self.to_int(), '-', other.to_int())
+        z = copy(other)
+        z.sign = not z.sign
+        #print(self.to_int(), '+', z.to_int(), 'sub')
+        return self + z
+
     def add_like_signs(self, other):
         '''
         Add a BigInt of the same sign as self.
@@ -71,6 +93,108 @@ class BigInt:
         result.digits = out
         return result
 
+    def add_unlike_signs(self, other):
+        '''
+        Add a BigInt of unlike sign as self.
+        '''
+        assert self.sign != other.sign
+
+        # So we have -a and +b
+        #         or +a and -b
+
+        if self.sign:
+            a, b = self, other
+        else:
+            b, a = self, other
+
+        #print(a.to_int(), '+', b.to_int(), 'add_unlike_signs')
+
+        # So now we have:
+        #     a + (-b) == a - b
+
+        # I don't know how to subtract a larger (abs) number
+        #                        from a smaller (abs) one
+        # However:
+        #
+        # a - b == -(b - a)
+        #
+        # I.e. 9 - 17 == -(17 - 9)
+
+
+        #if abs(a) < abs(b):
+        if not a.abs_gt_abs(b):
+            #print(f'abs({a.to_int()}) < abs({b.to_int()})')
+            x = b._subtract_smaller(a)
+            x.sign = not x.sign
+            return x
+        #print(f'abs({a.to_int()}) > abs({b.to_int()})')
+        return a._subtract_smaller(b)
+
+    def _subtract_smaller(self, other):
+        assert self.abs_gt_abs(other)
+        out = []
+        carry = 0
+        Z = zip_longest(
+            self.digits,
+            other.digits,
+            fillvalue=zero,
+            )
+        #P(list(Z))
+        for a, b in Z:
+            carry, digit = a.sub_with_carry(b, carry)
+            out.append(digit)
+        if carry:
+            out.append(one)
+        result = BigInt()
+        result.sign = self.sign
+        result.digits = out
+        return result
+
+    def abs_gt_abs(self, other):
+        a_len, b_len = len(self.digits), len(other.digits)
+        if a_len > b_len: return True
+        if a_len < b_len: return False
+        # a_len == b_len
+        if not a_len:  # a == b == 0
+            return False
+        return self.digits[-1] > other.digits[-1]
+
+
+##        result = BigInt()
+##        result.sign = self.sign
+##        result.digits = (
+##            self.subtract_digits(other)
+##            if self.sign else
+##            other.subtract_digits(self)
+##            )
+##        return result
+
+##    def subtract_digits(self, other):
+##        return []
+
+##def _sort_key(list_of_obint):
+##    n = len(list_of_obint)
+##    last = list_of_obint[-1] if n else None
+##    return n, zero
+
+##def subtract_list_of_obints(A, B):
+##    L = [A, B]
+##    K = sorted(L, key=_sort_key)
+##    A, B = K
+##    swapped = L != K
+##    carry = 0
+##    out = []
+##    for a, b in zip_longest(A, B, fillvalue=zero):
+##        carry, digit = a.sub_with_carry(b, carry)
+##        out.append(digit)
+##    if carry:
+##        out.append(one)
+##    result = BigInt()
+##    result.sign = self.sign
+##    result.digits = out
+##    return result
+
+
 
 class OberonInt:
     '''
@@ -94,6 +218,24 @@ class OberonInt:
             assert not z, repr(z)
         return c, digit
 
+    def sub_with_carry(self, other, carry):
+        '''
+        In terms of single base-10 skool arithmetic:
+
+        a, b in {0..9}
+        carry in {0..1}
+
+        0 - 9 - 1
+
+        9 + 9 + 1 = 18 + 1  = 19
+        aka       = 1,(8+1) = 1, 9
+        '''
+        c, digit = self - other
+        if carry:
+            z, digit = digit - one
+            assert not z, repr(z)
+        return c, digit
+
     def __init__(self, initial=0):
         assert is_i32(initial)
         self.value = initial
@@ -211,6 +353,33 @@ class BigIntTest(unittest.TestCase):
         t = z.to_int()
         self.assertEqual(t, n + m)
 
+    def test_Addition_of_unlike_signs(self):
+        n = 12345678901234567898090123445678990
+        m = -901234567898090
+        x = BigInt(n)
+        y = BigInt(m)
+        z = x + y
+        t = z.to_int()
+        self.assertEqual(t, n + m)
+
+    def _test_invert(self):
+        n = 7 * (2**16)
+        x = BigInt(n)
+        y = x.negate()
+        print()
+        print(y.to_int(), bin(y.to_int()), y.digits)
+        print(x.to_int(), bin(x.to_int()), x.digits)
+        print()
+        print(x + y)
+
+    def test_Subtraction(self):
+        n = 12345678901234567898090123445678990
+        m = 901234567898090
+        x = BigInt(n)
+        y = BigInt(m)
+        z = x - y
+        t = z.to_int()
+        self.assertEqual(t, n - m)
 
 
 if __name__ == '__main__':
@@ -225,5 +394,3 @@ if __name__ == '__main__':
 ##            raise ValueError(f'too big: {initial!r}')
 ##        if initial < -2**31:
 ##            raise ValueError(f'too small: {initial!r}')
-
-