let x1, x2, x3, x4 be object ; :: thesis: {x1,x1,x1,x1,x2,x3,x4} = {x1,x2,x3,x4}
thus {x1,x1,x1,x1,x2,x3,x4} = {x1,x1,x2,x3,x4} by Th40
.= {x1,x2,x3,x4} by Th32 ; :: thesis: verum