let x1, x2, x3, x4, x5, x6 be set ; :: thesis: {x1,x1,x2,x3,x4,x5,x6} = {x1,x2,x3,x4,x5,x6}
thus {x1,x1,x2,x3,x4,x5,x6} = {x1,x1} \/ {x2,x3,x4,x5,x6} by Th57
.= {x1} \/ {x2,x3,x4,x5,x6} by Th69
.= {x1,x2,x3,x4,x5,x6} by Th51 ; :: thesis: verum