let x1, x2, x3, x4, x5 be set ; :: thesis: {x1,x1,x1,x1,x2,x3,x4,x5} = {x1,x2,x3,x4,x5}
thus {x1,x1,x1,x1,x2,x3,x4,x5} = {x1,x1,x2,x3,x4,x5} by Th81
.= {x1,x2,x3,x4,x5} by Th73 ; :: thesis: verum