let x1, x2, x3, x4, x5, x6 be object ; :: 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 Th17
.= {x1} \/ {x2,x3,x4,x5,x6} by Th29
.= {x1,x2,x3,x4,x5,x6} by Th11 ; :: thesis: verum