@Section
   @Title { "@StartHSpan","@StartVSpan", "@StartHVSpan", "@HSpan", and "@VSpan" }
   @Tag { hspan }
@Begin
@PP
starthspan.  @Index { @@StartHSpan symbol }
startvspan.  @Index { @@StartVSpan symbol }
starthvspan.  @Index { @@StartHVSpan symbol }
hspan. @Index { @@HSpan symbol }
vspan. @Index { @@VSpan symbol }
These symbols work together to produce spanning columns and rows in
a more flexible way than is possible in practice with @Code "//" and
{@Code "||"}.  An object
@ID @Code "@StartHSpan object"
causes @Code object to be printed, but occupying all the horizontal space
to the right on the row mark on which it lies up to and including the rightmost
@@HSpan symbol on that mark not preceded by @@StartHVSpan, @@StartHSpan,
@@StartVSpan, or @@VSpan.  The column mark of this spanning object is not
constrained to align with any of the column marks of the columns it spans.
@PP
If there is no @@HSpan symbol anywhere to the right of @@StartHSpan, then
the object spans only its own column.  This means that it occupies that
column as usual but its mark is not constrained to align with those of the
other objects in the column.
@PP
Similarly, the @@StartVSpan symbol causes its object to occupy all
the vertical space below it on the column mark on which it lies, down to and
including the bottommost @@VSpan symbol on that mark not preceded by a
@@StartHVSpan, @@StartHSpan, @@StartVSpan, or @@HSpan; and if there 
is no @@VSpan symbol anywhere below it on that mark, then the object
spans only its own row, occupying its row but with its mark not constrained
to align with the row mark.
@PP
The @@StartHVSpan symbol combines the effects of @@StartHSpan and
@@StartVSpan, allowing an object to span both columns and rows
simultaneously.  For example, in
@ID @Code {
"@StartHVSpan x  |  | @HSpan"
"/"
"@VSpan   |  |"
}
the object @Code x will occupy a rectangular area spanning three columns,
two rows, and the gaps between them.
@PP
The objects lying in the region spanned should all be empty, or the
@@HSpan and @@VSpan symbols can be used to document the spanning that
is occurring.  At present there may be no galley targets or recursive symbols
within the right parameter of @@StartHSpan, @@StartVSpan, or
@@StartHVSpan.  However, the right parameter may otherwise be an
arbitrary object, including paragraphs of text that require breaking.
@PP
If the right parameter of @@StartHSpan, @@StartVSpan, or @@StartHVSpan
occupies more horizontal or vertical space than all of the spanned columns
or rows combined require, the extra space goes into the last spanned column
or row.  Overlapping spanning rows and columns are permitted.  Gaps spanned
by span objects are unbreakable (their @Code "u" indicator is set
automatically and cannot be revoked).
@End @Section
