let x1, x2, x3, x4 be object ; :: thesis: {x1,x2,x3,x4} = {x1,x4,x3,x2}
thus {x1,x2,x3,x4} = {x1} \/ {x2,x3,x4} by Th4
.= {x1} \/ {x4,x3,x2} by Th60
.= {x1,x4,x3,x2} by Th4 ; :: thesis: verum