let x1, x2, x3, x4, x5, x6, x7 be set ; :: thesis: [x1,x2,x3,x4,x5,x6,x7] = [[x1,x2,x3],x4,x5,x6,x7]
thus [x1,x2,x3,x4,x5,x6,x7] = [[[[[[x1,x2],x3],x4],x5],x6],x7] by Th78
.= [[[x1,x2],x3],x4,x5,x6,x7] by Th3
.= [[x1,x2,x3],x4,x5,x6,x7] by MCART_1:def 3 ; :: thesis: verum