for x1, x2, y1, y2 being set
for f being Function holds
( ( x1 = x2 implies (f +* (x1 .--> y1)) +* (x2 .--> y2) = f +* (x2 .--> y2) ) & ( x1 <> x2 implies (f +* (x1 .--> y1)) +* (x2 .--> y2) = (f +* (x2 .--> y2)) +* (x1 .--> y1) ) )