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 MCART_1:def 4
.= [[[[x1,x2],x3],x4],x5] by MCART_1:def 3 ; :: thesis: verum