let x1, x2, x3, x4 be object ; :: thesis: {x1,x2,x3,x4} = {x4,x3,x2,x1}
thus {x1,x2,x3,x4} = {x3,x4,x2,x1} by Th74
.= {x4,x3,x2,x1} by Th65 ; :: thesis: verum