Source — Euclid's Elements, encoded as an rrxiv paper

V.17 Definition V.17

A ratio ex aequali arises when, there being several magnitudes and another set equal to them in multitude which taken two and two are in the same proportion, as the first is to the last of the first magnitudes, so is the first to the last of the second magnitudes.

lines 74–74 in main.tex