let x1, x2, x3, x4, x5, x6, x7 be object ; :: thesis: {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 Th3
.= ({x1,x2,x3,x4} \/ {x5,x6}) \/ {x7} by XBOOLE_1:4
.= {x1,x2,x3,x4,x5,x6} \/ {x7} by Th14 ; :: thesis: verum