let x1, x2, x3, x4, x5, x6 be object ; :: thesis: {x1,x2,x3,x4,x5,x6} = {x1,x2,x3,x4} \/ {x5,x6}
thus {x1,x2,x3,x4,x5,x6} = {x1,x2,x3} \/ {x4,x5,x6} by Lm4
.= {x1,x2,x3} \/ ({x4} \/ {x5,x6}) by Th2
.= ({x1,x2,x3} \/ {x4}) \/ {x5,x6} by XBOOLE_1:4
.= {x1,x2,x3,x4} \/ {x5,x6} by Th6 ; :: thesis: verum