R-10 All Subtraction Rules

(+) - (+)
Find the difference between the 2 numbers and  if the number you are subtracting from is larger and answer is (+).

(-) -(+)
Add the absolute values of 2 #'s make answer negative.
Justification: Because you add the same # of zero's as the second # because there are no (+) to take away. After you take (+) away, you total (-) and increase by the amount of the second #. 

(+)-(-)

* Add the two absolute values
*Why: Because you have to add zeros in order to take away negative
*Rewrite the problem into an addition problem. 

(-)-(-)
Change the #'s to their absolute value and find the difference. If the larger absolute value is first the answer is (-). If the larger absolute value is second, the answer is (+).
Justification: If you don't have enough (-) to take away, add pairs of zeros until you do. Then take away (-). Remaining is the answer.

How are the rules similar? Can you generalize?