char * sb(char *a, char *b, short n, char *res)
{
	if (!((n > 1 && n < 11) || n == 16))
	{
		printf("Invalid numeric system\n");
		return NULL;
	}
	if (a[0] == '0')
	{
		printf("Invalid a[0]!=0\n");
		return NULL;
	}
	//struct operation op;
	//buildoperation(a, b, n, res, &op, SUB);
	long alen = 0 - (a - strchr(a, '\0'));
	long blen = 0 - (b - strchr(b, '\0'));
	if (alen < blen)
	{
		printf("Invalid a>=b\n");
		return NULL;
	}
	else if (alen == blen)
	{
		for (long l = 0; l < alen; l++)
		{
			if (a[l] == b[l])
			{
				if (l == alen - 1)
				{
					res = "0\0";
					return res;
				}
				continue;
			}
			else if (a[l] > b[l])
			{
				break;
			}
			else
			{
				printf("Invalid a>=b\n");
				return NULL;
			}
		}
	}
	res[alen] = '\0';
	alen -= 1;
	blen -= 1;
	short m = 0;
	for ( ; alen >= 0; alen--, blen--)
	{
		short aa = (a[alen] >= '0' && a[alen] <= '9') ? (a[alen] - '0') : (a[alen] - 'A' + 10);
		short bb =
			(blen >= 0 ?
			((b[blen] >= '0' && b[blen] <= '9') ? (b[blen] - '0') : (b[blen] - 'A' + 10))
			: 0);
		if (m == 1)
		{
			if (aa == 0)
			{
				a[alen] = (n < 11) ? ('0' + n - 1) : ('A' + n - 10 - 1);
			}
			else
			{
				a[alen] =
				(n < 11) ?
				(a[alen] - 1)
				: ((a[alen] == 'A') ? '9' : (a[alen] - 1));
				m = 0;
			}
		}
		if (aa >= bb)
		{
			aa = aa - bb;
		}
		else
		{
			m = 1;
			aa = (n < 11) ? (n + aa) : (10 + aa);
			aa = aa - bb;
		}
		res[alen] = (aa < 10) ? ('0' + aa) : ('A' + aa - 10);
	}/*
	if (res[0] == '0')
	{
		alen = 0 - (res - strchr(res, '\0'));
		long l = 0;
		while (res[l] == '0')
		{
			l++;
		}
		for (long ll = 0; ll < alen; ll++)
		{
			res[ll] = res[ll + l];
		}
	}*/
	return res;
}