Today I learned

Union

> [:a, :b, :c].union([:b, :c, :d])
 => [:a, :b, :c, :d]
# or 
> [:a, :b, :c] | [:b, :c, :d]
 => [:a, :b, :c, :d]

“Union all” / Concat

[:a, :b].concat([:b, :c])
 => [:a, :b, :b, :c]
# or
[:a, :b] + [:b, :c]
 => [:a, :b, :b, :c]

Difference

> [:a, :b, :c].difference([:b, :c, :d])
 => [:a]
# or
[:a, :b, :c] - [:b, :c, :d]
 => [:a]

Intersect

> [:a, :b, :c] & [:b, :c, :d]
 => [:b, :c]