There is an interesting contradiction in the laws of writing Sifrei Torah, tefillin, and mezuzot. In the Shulchan Arukh 32:18, Maran discusses the letter, "mem," which is formed from two letters, a "nun" and a "vav."

On the on hand, he writes that if the sofer writes the letter "mem," but accidentally does not leave the left side of the "mem" open, i.e., it is completely closed all around, it is sufficient to erase the left side of the "mem," the "vav," and write it again. However, he cannot just carve out a bit of the letter" vav," as that would be a violation of the principle of hak tochot, which teaches that a letter must be formed by writing, and not by carving. Therefore, in this case, Maran allows the sofer to correct the letter "mem" without erasing and rewriting the entire letter. However, regarding the letter "resh," Maran writes that if the "resh" looks like a "dalet," the sofer must erase and rewrite the entire letter. In this case, why does the Shulchan arukh require the sofer to rewrite the entire letter?

The Taz explains that since the "mem" is formed from two letters, i.e., the "nun," and then the "vav," it is possible to keep the "nun" and rewrite just the "vav." However, since the "resh" is written in one stroke, if a mistake is made, the entire letter must be erased and rewritten.

Summary: Although the principle of hak tochot teaches that a letter must be written, and not carved out, when a letter is made from two separate letters and written in different strokes, it is sufficient to erase and then rewrite one of the letters, and then complete the letter.