let X1, X2, X3, X4 be set ; :: thesis: [:[:X1,X2:],X3,X4:] = [:X1,X2,X3,X4:]
thus [:[:X1,X2:],X3,X4:] = [:[:[:X1,X2:],X3:],X4:] by ZFMISC_1:def 3
.= [:X1,X2,X3,X4:] by Th37 ; :: thesis: verum