@Section
   @Title { {"@Common"}, {"@Rump"}, and "@Meld" }
   @Tag { rump }
@Begin
@PP
common.sym @Index { @@Common symbol }
rump.sym @Index { @@Rump symbol }
meld.sym @Index { @@Meld symbol }
The @@Common and @@Rump symbols compare two paragraph objects:
@ID @Code "{ Aardvark, 29 } @Common { Aardvark, 359 }"
If either parameter is not a paragraph object, it is converted into
a single-object paragraph first.  The result of @@Common is the
common prefix of the two paragraphs; that is, those initial objects
which are equal in the two paragraphs.  In the example above, the
result is {@Code "Aardvark,"}.  The result of @@Rump is that part of
the second object which is not included in @@Common; the result of
@ID @Code "{ Aardvark, 29 } @Rump { Aardvark, 359 }"
is {@Code "359"}.
@PP
If the two objects have nothing in common, the result of @@Common will
be an empty object and the result of @@Rump will be the second
object.  If the two objects are identical, the result of @@Common will
be the first object, and the result of @@Rump will be an empty object.
@PP
The only known use for @@Rump and @@Common is to implement merged index
entries (Section {@NumberOf sorted}).
@PP
The @@Meld symbol returns the minimum meld of two paragraphs, that
is the shortest paragraph that contains the two original paragraphs
as subsequences.  For example,
@ID @Code "{ Aardvark , 1 , 2 } @Meld { Aardvark , 2 , 3 }"
produces
@ID { Aardvark , 1 , 2 } @Meld { Aardvark , 2 , 3 }
The result is related to the well-known
longest common substring, in that the meld contains everything not in
the lcs plus one copy of everything in the lcs.  This function is
well-suited to melding complex index entries.  Note that there must be
a non-zero amount of space before each comma, otherwise each will become
part of the preceding word, and, since @@Meld compares the two paragraphs
word by word, the result will be different and less satisfactory.
@PP
Determining the values of all these symbols requires testing whether
one component of the first paragraph is equal to one component of the
second.  Since Version 3.25, the objects involved may be arbitrary
and Lout will perform the necessary detailed checking for equality;
previously, only simple words were guaranteed to be tested correctly.
@End @Section
