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 Th3
.= ({x1,x2,x3} \/ {x4,x5}) \/ {x6} by XBOOLE_1:4
.= {x1,x2,x3,x4,x5} \/ {x6} by Lm3 ; :: thesis: verum