let x1, x2, x3, x4, x5, x6, x7 be object ; {x1,x2,x3,x4,x5,x6,x7} = {x1,x2} \/ {x3,x4,x5,x6,x7}
thus {x1,x2,x3,x4,x5,x6,x7} =
{x1,x2,x3,x4} \/ {x5,x6,x7}
by Lm5
.=
({x1,x2} \/ {x3,x4}) \/ {x5,x6,x7}
by Lm2
.=
{x1,x2} \/ ({x3,x4} \/ {x5,x6,x7})
by XBOOLE_1:4
.=
{x1,x2} \/ {x3,x4,x5,x6,x7}
by Th8
; verum