Union of two sets or of two lists

The union operator is an infixed operator that finds the union of the elements of two sets or lists; the result will always be a set.

set[1,2,3,4] union set[5,6,3,4]

or:

⟦ 1,2,3,4,5,6 ⟧

[1,2,3] union [2,5,6]

⟦ 1,2,3,5,6⟧