APPENDCOLS
=LAMBDA(xs,
    LAMBDA(ys,
        LET(
            nx, COLUMNS(xs),
            colIndexes, SEQUENCE(1, nx + COLUMNS(ys)),
            rowIndexes, SEQUENCE(MAX(ROWS(xs), ROWS(ys))),

            IFERROR(
                IF(nx < colIndexes,
                    INDEX(ys, rowIndexes, colIndexes - nx),
                    INDEX(xs, rowIndexes, colIndexes)
                ),
                NA()
            )
        )
    )
)


CHARSROW
=LAMBDA(s,
    MID(s,
        SEQUENCE(1, LEN(s), 1, 1),
        1
    )
)


HEADCOL
=LAMBDA(xs,
    LET(REM, "The first item of each column in xs",

        INDEX(xs, 1, SEQUENCE(1, COLUMNS(xs)))
    )
)


HEADROW
=LAMBDA(xs,
    LET(REM, "The first item of each row in xs",

        INDEX(
            xs,
            SEQUENCE(ROWS(xs)),
            SEQUENCE(1, 1)
        )
    )
)


LASTCOL
=LAMBDA(xs,
    INDEX(
        xs,
        SEQUENCE(ROWS(xs), 1, 1, 1),
        COLUMNS(xs)
    )
)


SCANLCOLS
=LAMBDA(op,
    LAMBDA(a,
        LAMBDA(xs,
            APPENDCOLS(a)(
                LET(
                    b, op(a, HEADROW(xs)),

                    IF(2 > COLUMNS(xs),
                        b,
                        SCANLCOLS(op)(b)(
                            TAILROW(xs)
                        )
                    )
                )
            )
        )
    )
)


TAILCOL
=LAMBDA(cols,
    LET(REM, "The tail of each column in the grid.",

        INDEX(
            cols,
            SEQUENCE(ROWS(cols) - 1, 1, 2, 1),
            SEQUENCE(1, COLUMNS(cols))
        )
    )
)


TAILROW
=LAMBDA(xs,
    LET(REM,"The tail of each row in the grid",
        n, COLUMNS(xs) - 1,

        IF(0 < n,
            INDEX(
                xs,
                SEQUENCE(ROWS(xs), 1, 1, 1),
                SEQUENCE(1, n, 2, 1)
            ),
            NA()
        )
    )
)