let X1, X2, X3, X4, X5 be set ; :: thesis: [:X1,X2,X3,X4,X5:] = [:[:X1,X2,X3:],X4,X5:]
thus [:X1,X2,X3,X4,X5:] = [:[:[:[:X1,X2:],X3:],X4:],X5:] by TH9
.= [:[:[:X1,X2:],X3:],X4,X5:] by ZFMISC_1:def 3
.= [:[:X1,X2,X3:],X4,X5:] by ZFMISC_1:def 3 ; :: thesis: verum