Changes

<br />

<br />

Let <math>S = \{\{x,y\}\ |\ x \in X \and y \in Y\}</math>.  The vertical bar means "such that", and the epsilon-like symbol means "is an element of".  So this means that S is the collection of sets<ref>Mathematicians don't like to use the phrase "set of sets", since this can lead to some unpleasant paradoxes.  So they say "collection of sets."  A "collection" is really just a set.</ref> which are pairs x and y such that x is in X and y is in Y.

Let <math>S = \{\{x,y\}\ |\ x \in X \and y \in Y\}</math>.  The vertical bar means "such that", and the epsilon-like symbol <math>\in</math> means "is an element of".  So this means that S is the collection of sets<ref>Mathematicians don't like to use the phrase "set of sets", since this can lead to some unpleasant paradoxes.  So they say "collection of sets."  A "collection" is really just a set.</ref> which are pairs x and y such that x is in X and y is in Y.

<br />

<br />