theorem :: ENUMSET1:81

for x1, x2, x3, x4, x5, x6, x7, x8, x9 being object holds {x1,x2,x3,x4,x5,x6,x7,x8,x9} = {x1,x2,x3,x4,x5} \/ {x6,x7,x8,x9}