let x1, x2, x3, x4, x5, x6, x7 be set ; :: thesis: {x1,x1,x2,x3,x4,x5,x6,x7} = {x1,x2,x3,x4,x5,x6,x7}
thus {x1,x1,x2,x3,x4,x5,x6,x7} = {x1,x1} \/ {x2,x3,x4,x5,x6,x7} by Th63
.= {x1} \/ {x2,x3,x4,x5,x6,x7} by Th69
.= {x1,x2,x3,x4,x5,x6,x7} by Th56 ; :: thesis: verum