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 Th7
.=
({x1,x2,x3,x4} \/ {x5}) \/ {x6,x7,x8,x9}
by XBOOLE_1:4
.=
{x1,x2,x3,x4,x5} \/ {x6,x7,x8,x9}
by Th10
; verum