The lemma in his note is Oratesque, the reading of Daniel Heinsius' edition.
The method of exhaustion used, for the purpose of proof by reductio ad absurdum, the lemma proved in Eucl.
His apparatus is the first to supply a lemma for each variant reading reported, and is clear and easy to read.
1560s, first in mathematics, from Greek lemma (plural lemmata) "something received or taken; an argument; something taken for granted," from root of lambanein "to take" (see analemma).
logic
A result already proved, which is needed in the proof of some further result.
(1995-03-25)