let x1, x2, x3, x4 be object ; :: thesis: {x1,x2,x3,x4} = {x2,x1,x3,x4}
thus {x1,x2,x3,x4} = {x1,x2,x3} \/ {x4} by Th6
.= {x2,x1,x3} \/ {x4} by Th58
.= {x2,x1,x3,x4} by Th6 ; :: thesis: verum