let x1, x2, x3, x4, x5, x6, x7, x8, x9 be object ; {x1,x2,x3,x4,x5,x6,x7,x8,x9} = {x1,x2,x3} \/ {x4,x5,x6,x7,x8,x9}
thus {x1,x2,x3,x4,x5,x6,x7,x8,x9} =
{x1,x2,x3,x4} \/ {x5,x6,x7,x8,x9}
by Lm9
.=
({x1,x2,x3} \/ {x4}) \/ {x5,x6,x7,x8,x9}
by Th6
.=
{x1,x2,x3} \/ ({x4} \/ {x5,x6,x7,x8,x9})
by XBOOLE_1:4
.=
{x1,x2,x3} \/ {x4,x5,x6,x7,x8,x9}
by Th11
; verum