**        of the page straight up to this node.
    **   mo:  "merge-out".  If non-zero, this is one more than the x-coordinate
    **        for the upward portion of a merge arrow.  The merge arrow goes up
    **        to the row identified by mu:.  If this value is zero then
    **        node has no merge children and no merge-out line is drawn.
    **   mu:  The id of the row which is the top of the merge-out arrow.
    **    u:  Draw a thick child-line out of the top of this node and up to
    **        the node with an id equal to this value.  0 if it is straight to
    **        the top of the page, -1 if there is no thick-line riser.
    **    f:  0x01: a leaf node.
    **   au:  An array of integers that define thick-line risers for branches.
    **        The integers are in pairs.  For each pair, the first integer is
    **        is the rail on which the riser should run and the second integer
    **        is the id of the node upto which the riser should run.
    **   mi:  "merge-in".  An array of integer x-coordinates from which
    **        merge arrows should be drawn into this node.  If the value is