Complex sentence: Section ✸ 11 applies this symbolism to two variables, i.e. ⊃ "x", ⊃ "y", ⊃ "x, y" could all appear in a single formula.
Split into simpler sentences: Section ✸ 11 applies this symbolism to two variables. Thus the following notations: ⊃ "x", ⊃ "y", ⊃ "x, y" could all appear in a single formula.