Variables2[expr] is like Variables, but it
also works on rules and equalities as well as lists thereof.
Variables2 always applies Union to the
output.
Some cases where Variables2 is much more useful than
Variables
Variables[{a -> x1 + y1, b -> x2 + y2}]\{\}
Variables2[{a -> x1 + y1, b -> x2 + y2}]\{a,b,\text{x1},\text{x2},\text{y1},\text{y2}\}
Variables[a + b == c + d]\{\}
Variables2[a + b == c + d]\{a,b,c,d\}