7
range
m:
    power-of-2
    range1
    m:
        "/"
        swap
        ljust
        "\\"
        catenate #(list+list)
        @7
        double
        center
    }
    2
    step
    swap
    lines
        dup
        catenate #(list+list)
    m2
    catenate #(list+list)
    unlines