STAM must be written (an additive process) and not hollowed out (a subtractive process). This means that the final form of each letter can not be created by removing some extraneous part. A very simple example would be where the Sofer wrote a “ה” instead of a “ד”: If he would simply erase the left leg of the ה, all that would be left is a ד, but it would pasul because it was created by chok tochos. The proper way to fix the letter (according to most poskim) would be to erase both legs of the ה (leaving only the top of the letter), and then rewrite the right leg. Of course, this solution is only possible if there is no issue of writing shelo kisidran.
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STAM 101 introduction & index