let x1, x2, x3 be set ; :: thesis: {x1,x1,x1,x1,x1,x1,x2,x3} = {x1,x2,x3}
thus {x1,x1,x1,x1,x1,x1,x2,x3} = {x1,x1,x2,x3} by Th90
.= {x1,x2,x3} by Th71 ; :: thesis: verum