let x1, x2, x3, x4, x5, x6, x7, x8, x9 be object ; :: thesis: {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 Lm2
.= {x1,x2} \/ ({x3,x4} \/ {x5,x6,x7,x8,x9}) by XBOOLE_1:4
.= {x1,x2} \/ {x3,x4,x5,x6,x7,x8,x9} by Th17 ; :: thesis: verum