let x1, x2, x3 be set ; :: thesis: {x1,x2,x3} = {x1,x2} \/ {x3}
thus {x1,x2,x3} = {x1} \/ {x2,x3} by Th42
.= {x1} \/ ({x2} \/ {x3}) by Th41
.= ({x1} \/ {x2}) \/ {x3} by XBOOLE_1:4
.= {x1,x2} \/ {x3} by Th41 ; :: thesis: verum