APPEND
=LAMBDA(xs,
    LAMBDA(ys,
        LET(
            nx, ROWS(xs),
            rowIndexes, SEQUENCE(nx + ROWS(ys)),
            colIndexes, SEQUENCE(
                1,
                MAX(COLUMNS(xs), COLUMNS(ys))
            ),
            IF(rowIndexes <= nx,
                INDEX(xs, rowIndexes, colIndexes),
                INDEX(ys, rowIndexes - nx, colIndexes)
            )
        )
    )
)


ENUMFROMTO
=LAMBDA(a,
    LAMBDA(z,
        SEQUENCE(1 + z - a, 1, a, 1)
    )
)


FOLDL
=LAMBDA(op,
    LAMBDA(a,
        LAMBDA(xs,
            IF(
                2 > ROWS(xs),
                op(a)(xs),
                FOLDL(op)(
                    op(a)(
                        HEAD(xs)
                    )
                )(
                    TAIL(xs)
                )
            )
        )
    )
)


HEAD
=LAMBDA(xs,
    INDEX(xs, 1, SEQUENCE(1, COLUMNS(xs)))
)


TAIL
=LAMBDA(xs,
    INDEX(
        xs,
        SEQUENCE(ROWS(xs) - 1, 1, 2, 1),
        SEQUENCE(1, COLUMNS(xs))
    )
)