let x1, x2, x3, x4 be set ; :: thesis: {x1,x1,x2,x3,x4} = {x1,x2,x3,x4}
thus {x1,x1,x2,x3,x4} = {x1,x1} \/ {x2,x3,x4} by Th48
.= {x1} \/ {x2,x3,x4} by Th69
.= {x1,x2,x3,x4} by Th44 ; :: thesis: verum