@Section
   @Title { "@Case" }
   @Tag { case }
@Begin
@PP
case.sym @Index { @@Case symbol }
yield.sym @Index { @@Yield symbol }
The @@Case symbol selects its result from a list of alternatives,
depending on a tag:
@ID @Code {
"@Day @Case {"
"    { 1 21 31 }  @Yield  st"
"    { 2 22 }  @Yield  nd"
"    { 3 23 }  @Yield  rd"
"    else  @Yield th"
"}"
}
In this example the result will be st if @Code "@Day" is 1, 21, or 31,
and nd if @Code "@Day" is 2 or 22, etc.  The effect is similar to
accessing a database, though in a more compact form.  The right parameter
is a sequence of @@Yield symbols, each with a left parameter whose value is
a sequence of one or more juxtapositions of simple words, and a right
parameter which may be any object.
@PP
We first describe the behaviour when the value of the left parameter of
@@Case is a juxtaposition of one or more simple words.  Then the result of
the @@Case is the right parameter of the first @@Yield whose left parameter
contains either the value of the left parameter of the @@Case, or the special
value {@Code else}.  If there is no such @@Yield it is an error.
@PP
When the left parameter of @@Case is not a juxtaposition of simple
words, the result is the right parameter of the first @@Yield whose
left parameter is {@Code else}, or an error otherwise.  This permits
examples like
@ID @Code {
"@RunningTitle @Case {"
"    dft @Yield @Title"
"    else @Yield @RunningTitle"
"}"
}
where a running title is returned unless it has the value {@Code dft}
(which presumably means that no running title was supplied), in which
case an ordinary title is returned instead.
@PP
When a receptive symbol is placed within a @@Case, it should be included
in each alternative, since otherwise Basser Lout may become confused when
trying to predict whether the symbol will be a part of the result or
not.  Alternatively, if it can be guaranteed that the receptive symbol
will never be searched for when the cases that it does not lie within
are selected, that is all right too.
@End @Section