fuzzy or

DEFDBL A-Z
a = RND
b = RND
c = RND

'{{a,b},c}
s = a + b - a * b
s = s + c - s * c
PRINT s

'{{a,c},b}
s = a + c - a * c
s = s + b - s * b
PRINT s

'{a,b,c} explicit
s = a + b + c - (a * b + b * c + c * a) + a * b * c
PRINT s