SETUNI: Perform a set union operation on two positive integer sets
SETUNI(A,B,C), will assign to set A: B u C,
u: denotes set union

syntax: SETUNI(target_letter, set_letter1, set_letter2)

Performs a set union operation on the sets with the letter variable <set_letter1> and <set_letter2> which refers to letter variable ($A...$Z) then puts the results from the set union operation into <target_letter> set.