it's to solve the "No margin" situations:
if you leave it just as "margin" it gives you an error on the first 3 examples, in which there is no margin.
putting it as a keyword argument prevents that error by making the "default value" of margin = 0
it's also 0 because "abs(a-b) can never be negative"
-> by writing this comment I realized it could be better to put "margin = -1" to prevent the case of "abs(a-b) == 0" that would result in an error.