SETXDIF: Perform a symmetric set difference operation on two positive integer
sets. SETXDIF(A,B,C), will assign to set A: (B u C) - (B n C),
u: denotes set union,
n: denotes set intersection

syntax: SETXDIF(target_letter, set_letter1, set_letter2)

Performs a symmetric set difference 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 symmetric set difference operation into <target_letter> set.