Amud 33a

תוס' ד"ה אין

Craftsmen are not permitted to stand up, etc. - אין בעלי אומניות רשאין לעמוד כולי

Summary:
רשאין can mean permitted, when the אומנים are employees, or it can mean obligated if the אומנים are working on their own.

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תוס' ד"ה שני

Two fifths - שני חומשין

Summary:
A חומש does not mean a fifth but rather one of the five ספרים in תהלים.

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תוס' ד"ה שמא

Perhaps he was sitting and thinking about them, etc. - שמא בהן יושב ומהרהר כולי

Summary:
They would be excused for not standing up, no matter what they were learning, however רבי knew (through סוד ה' ליראיו) what they were studying.

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