This is some undisbutable logic:
"Right", he said uncertainly. His mind was grinding through the problem. She was a witch. Just lately there´d been a lot of gossip about witches being bad for your health. He´d been told not to let witches pass, but no-one had said anything about apple sellers. Apple sellers were not a problem. It was witches that were the problem. She´d said she was an apple seller and he wasn´t about to doubt a witch´s word.
Feeling happy with this application of logic, he stood to one side and gave an expansive wave.
"Pass, apple seller," he said.
Magrat just pulled off the traditional apple seller gambit, which has
[...] never worked more than once in the entire history of witchcraft [...]
Well done, Magrat. I´m loving this book so far.