let x1, x2, x3, x4 be object ; :: thesis: {x1,x2,x3,x4} = {x3,x2,x4,x1}
thus {x1,x2,x3,x4} = {x3,x2,x1,x4} by Lm8
.= {x3,x2,x4,x1} by Th61 ; :: thesis: verum