: 33matrix? ( seq -- ? ) 8 swap bounds-check? ; ERROR: not-33matrix seq ; :: det ( matrix -- x ) matrix 33matrix? not [ matrix not-33matrix ] [ matrix { 0 4 8 } nths product matrix { 1 5 6 } nths product matrix { 2 3 7 } nths product + + matrix { 0 5 7 } nths product matrix { 1 3 8 } nths product matrix { 2 4 6 } nths product + + - ] if ;