@Section
   @Title { "@NotRevealed" }
   @Tag { notrevealed }
@Begin
@PP
The @@NotRevealed symbol
notrevealed.sym @Index { @@NotRevealed symbol }
exerts fine control over the process of expanding receptive symbols.  It
may appear only within the body of a definition, immediately following
the name of a receptive symbol.  For example:
@ID @OneRow @Code {
"def A { @Galley }"
""
"def B { @Galley }"
""
"def ABList"
"{"
"    A"
"    //  B @NotRevealed"
"    //  ABList"
"}"
}
The meaning is that the symbol immediately preceding @@NotRevealed, @Code B
in this example, is not revealed to galleys which encounter @Code "ABList"
while searching for targets; to such galleys it appears that @Code "ABList"
contains @Code A only, not {@Code B}, hence only galleys targeted to
@Code A will expand {@Code "ABList"}.  However, after @Code "ABList"
is expanded by such a galley, @Code B will be available as a target in
the usual way.
@PP
Apart from this meaning, @@NotRevealed has no effect at all, and the
body of the definition may be understood by deleting @@NotRevealed and
any preceding space.  Thus, the symbol preceding @@NotRevealed may have
named and right parameters in the usual way; these would follow after
the @@NotRevealed symbol.
@PP
This symbol was introduced to overcome a problem with floating figures
treated as displays.  It turned out to be essential to specify the
layout of a column (in part) as
@ID @OneRow @Code {
"@BodyTextPlace"
"//  @FigurePlace"
"//  @BodyTextPlace"
"//  @FigurePlace"
"//  @BodyTextPlace"
"..."
}
so that figures could alternate with body text down the column.  However,
some means was needed to ensure that in the absence of any figures there
could only be one @Code "@BodyTextPlace" in the column, since otherwise
various problems arose, for example the @Code "@NP" symbol merely causing
a skip from one @Code "@BodyTextPlace" to the next in the same column,
rather than to the first in the next column.  Also, without this feature
the optimal page breaker's attempts to end a column early would be
frustrated by Lout then discovering that plenty of space existed at a
following @Code "@BodyTextPlace" in the same column.  The solution is
based on @Code "ABList" above; each occurrence of @Code "@BodyTextPlace"
after a @Code "@FigurePlace" is not revealed in the enclosing definition,
and so cannot be found by body text galleys unless a figure has
previously attached to the preceding {@Code "@FigurePlace"}.
@End @Section