@Section
   @Tag { underline }
   @Title { "@Underline" }
@Begin
@PP
underline.sym @Index { @@Underline symbol }
The @@Underline symbol underlines its right parameter, but only if that
parameter is a word or a paragraph:
@ID @Code "We @Underline { really do } mean this."
produces
@ID { We @Underline { really do } mean this. }
It is not possible to underline an arbitrary object using this symbol;
the @@Underline symbol will be ignored if this is attempted.
@PP
It is very easy to @I define a symbol which will underline an arbitrary
object, using the @@Graphic symbol.  This raises the question of why
@@Underline is needed at all.  The answer is that @@Underline has two
properties that distinguish it from symbols based on @@Graphic.
@PP
First, when @@Underline both contains a paragraph and is used within a
paragraph, as in the example above, the inner and outer paragraphs are
merged into one, permitting the underlined text to break over several
lines.  This is how the @@Font symbol works too, but symbols based on
@@Graphic do not permit this merging.
@PP
Second, Adobe font files specify the correct position and thickness of
underlining for each font, and the @@Underline symbol follows these
specifications.  The font used is the font of the first object underlined,
if it is a simple word, or else the font of the enclosing paragraph.
@PP
The colour of the underline is usually the same as the colour of the
text being underlined, but this can be changed using the
@@SetUnderlineColour symbol (Section {@NumberOf underline_colour}).
@End @Section