let x1, x2, x3, x4, x5, x6, x7, x8, x9 be set ; :: thesis: [x1,x2,x3,x4,x5,x6,x7,x8,x9] = [[x1,x2,x3],x4,x5,x6,x7,x8,x9]
thus [x1,x2,x3,x4,x5,x6,x7,x8,x9] = [[[[[[[[x1,x2],x3],x4],x5],x6],x7],x8],x9] by Th46
.= [[[x1,x2],x3],x4,x5,x6,x7,x8,x9] by MC340
.= [[x1,x2,x3],x4,x5,x6,x7,x8,x9] by MCART_1:def 3 ; :: thesis: verum