Is 'chak tochot' (carving out) prohibited for correcting letters in tefillin, and what corrections are forbidden for this reason?

Synopsis

All authorities agree chak tochot is prohibited for tefillin. Corrections that create the letter's form by removing ink rather than writing it are forbidden. This includes fixing a dalet that should be a reish, a bet that should be a kaf, and a closed mem. The requirement is 'v'katav' — writing, not carving.

Early Acharonim
Lenient
Strict
Lesser cited
Major authority

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