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 Th6
.= {x1,x2,x3} \/ ({x4} \/ {x5,x6,x7}) by XBOOLE_1:4
.= {x1,x2,x3} \/ {x4,x5,x6,x7} by Th4 ; :: thesis: verum