Sequence indexes are non-negative, not natural (0 is not a natural number).

Reported by Daniel May <mayds@ecn.purdue.edu>.

De-tabified everywhere.

Doc/ref/ref3.tex

@@ -227,7 +227,7 @@ and \code{z.imag}.
 \end{description} % Numbers
 
 \item[Sequences]
-These represent finite ordered sets indexed by natural numbers.
+These represent finite ordered sets indexed by non-negative numbers.
 The built-in function \function{len()}\bifuncindex{len} returns the
 number of items of a sequence.
 When the length of a sequence is \var{n}, the