The NEXT command is one of a series of directional commands that
    allows you to traverse a program's structure. The initial NEXT
    command displays the first traverse subtree, and positions an
    arrow to point to the bucket with the most data points. The
    corresponding source lines for that portion of the program are
    displayed alongside the plot. Subsequent NEXT commands move the
    arrow to the bucket with the next most data points (the next
    leaf), and so on. Other traverse commands are FIRST, BACK, and
    CURRENT.