Borrow (source code)
= Borrow
{title2=$b_i$}
In positional <subtraction>, a higher place supplies one unit of its place value when the current place cannot subtract the requested digit. In <binary representation>, <subtraction> of one, a low <bit> toggles at every operation and the borrow continues through each zero <bit>, stopping at the first one <bit>. Thus decrementing a <binary counter> toggles position $i$ exactly when all less significant <bits> were zero. This is the downward-counting counterpart of a <carry> propagating through one <bits> during incrementing.
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