Sum
A form of union construction on sets that keeps the members of the
respective sets R and S separate is called disjoint union (or sum):
R + S= {(zero, x) | x} U {(one,y)| y ?S}
“tags” to preserve origin of element
inR(x) = (zero,x) (for x ?R)
inS(x) = (one,y) (for y ?S)