subtracting one from a column is the same as taking one from the bottom, not from the top, don't think of it as adding one from which we can then subtract. Its' not that. need to subtract one from the bottom row.

Correct that. Need to subtract from that column, so need to in fact borrow
from that column .... keep going across until reach a one from which we can
turn it into a zero.

but still, we don't add that one to the column. 0 -1 is still 1, but do the
borrowing as specified above.