What does the ⊇ symbol mean?

Question

On the attached picture there is a symbol that I do not understand. To understand additive functional dependency, I need to know what a symbol means. Please advice?

This is the symbol where he says: "Suppose that X ⊇ Y and that ..."

⊇ =?

Thank!

Answer 1

means "X is a superset of Y". The symbol of the subset is the same, but flips horizontally.

Answer 2

, , X Y, .. Y X. , X Y. Y X, ,