@Section
   @Title { "@HCover" and "@VCover" }
   @Tag { hcover }
@Begin
@PP
hcover. @Index { @@HCover symbol }
vcover. @Index { @@VCover symbol }
The @@VCover symbol vertically scales its right parameter so that it
covers every object that shares its row mark.  For example,
@ID @Code "@VCover ( 45d @Rotate Hello @VCover )"
produces
@ID @ShowHMark { @VCover ( 45d @Rotate Hello @VCover ) }
The row mark has been added to show clearly where it lies.  This should be
compared with
@ID @Code "@VScale ( 45d @Rotate Hello @VScale )"
which produces
@ID @ShowHMark { @VScale ( 45d @Rotate Hello @VScale ) }
Scaling abandons mark alignment and so is able to exactly cover the
rest of the row, whereas covering preserves mark alignment and so
is obliged in general to more than cover the rest of the row.
@PP
If the parameter of @@VCover has zero vertical size, this is taken
to mean that covering is not required after all and the @@VCover is
silently ignored.  If however the parameter has non-zero size above
the mark but zero size below, or vice versa, this is taken to be an error
since scaling cannot make the parameter cover the rest of the row.
@PP
@@HCover is similar, horizontally covering all objects that share
its column mark.  Neither symbol works well near galley targets,
because the scale factor to apply is determined before any galley
flushing takes place.
@End @Section