I have added some notes on multiple columns to my discussion of 
footnotes.  The excerpt is attached.

Peter
	<s3 title="Multiple columns and footnotes">
	  <note>
	    A possible method for multi-column layout and balancing
	    with footnotes, using a galley-based approach.
	  </note>
	  <p>
	    This note assumes a galley, as discussed above, flowing
	    text with footnotes and possibly other blocks into a
	    possibly multi-column area.  The logic of flowing into
	    multiple columns is trivially applied to a single column.
	    The galley is manipulated primarily within the context of
	    the hypothetical <em>layout tree</em>.
	  </p>
	  <p>
	    Associated with the galley are two sets of data.
	    One contains the maps of all "natural" break-points and
	    the of all hyphenation break-points.  This set is
	    constructed at the time of construction of the galley and
	    is a constant for a given galley.  The second contains
	    dynamic data which represents one possible attempt to lay
	    out the galley.  There may be multiple sets of such data
	    to reflect varying attempts.  The data of this set are,
	    essentially, representations of line-areas, with the supporting
	    information necessary to determine these line-areas.
	  </p>
	  <p>
	    The line-area data includes the boundaries within the
	    galley of each line-area, the boundaries of each column
	    and the boundaries of the "page", or main area.  When a
	    line-area boundary occurs at a hyphenation point, a
	    "virtual hyphen" is assumed and accounted for in the
	    i-p-d.  As mentioned, individual footnote galleys will
	    hang from the parent galley.  The associated data of the
	    footnote galleys is similar: a once-only break-points map,
	    and one or more line-area maps.  No column boundaries are
	    required, but a page boundary is required at the end of
	    the last footnote or where a footnote breaks across a page
	    boundary.
	  </p>
	  <p>
	    A number of b-p-d values are also maintained.  For each
	    line-area, the b-p-d, the main area b-p-d increment, the
	    footnote b-p-d increment and the footnote's page-related
	    b-p-d increment are required.  The main-area b-p-d
	    increments for any particular line-area are dependent on
	    the column position of the line-area.  Total b-p-d's are
	    also kept: total footnote b-p-d, total main area b-p-d,
	    and totals for each column.
	  </p>
	</s3>
	<s3 title="Balancing columns">
	  <p>
	    Columns are balanced dynamically in the galley preliminary
	    layout.  While the galley retains its basic linear
	    structure, the accompanying data structures accomplish
	    column distribution and balancing.  As each line-area is
	    added, the columns are re-balanced; i.e., the various
	    boundary markers are adjusted.
	  </p>
	</s3>

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