let x1, x2, x3, x4 be object ; :: thesis: {x1,x2,x3,x4} = {x2,x4,x3,x1}
thus {x1,x2,x3,x4} = {x2,x3,x1,x4} by Lm7
.= {x2,x4,x3,x1} by Th63 ; :: thesis: verum