Bava Metzia 87B
When the tzad-hasheva rule rule is presented, it is not explained where the gmar-melocho qualification comes from. Well then it must be obvious -- from kerem it is the time of the kli, and from kama it's from chermesh. Ok, so this means that both psukim need to have these vital components (kli, chermesh), and thus neither is superfluous and one cannot be learned from the other. So how then does the gmara shortly thereafter (hahoo m'v'el kelyicha...) suggest that chermesh is not need to teach when it's ok to eat, because we already know that from kli?
Paul Davidowitz, Long Beach, NY 11561 USA
Paul, Baruch she'Kivanta! Your question is asked by the Tosfos ha'Rosh (DH ha'Hu mi'v'El Kelyicha).
He asks: How do we know for Kamah that the worker can eat only at Gemar Melachah? He writes that one cannot answer that we learn Kamah from Kerem, because Kerem is exempt from Challah, and thus the fact that the worker is allowed to eat in the vineyard may be because the Din of grapes is more lenient since they are exempt from Challah. Consequently, the Torah is more lenient and allows the worker to eat, while for Kamah the Torah might be more strict.
The Tosfos ha'Rosh answers that it is "Mistaber" (logical) to compare all different kinds of eating of the worker. Since we know that he can eat it only in the vineyard at Gemar Melachah, it is logical that this also applies to any scenario where the worker eats on the job; he may eat only at Gemar Melachah.
It transpires that it is a Sevara that once we know that the worker eats at Gemar Melachah for Kerem, this presumably applies for all kinds of work.
Kol Tuv,
Dovid Bloom