Palindrome Sentences
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Given a set of words, find the longest possible palin-
drome using only that set of words, with no repeated
words.

A palindrome is a phrase whose alphabetic characters
are in the same sequence when reversed, as in

   Madam, I'm Adam

or

   Able was I ere I saw Elba

or

   A man, a plan, a canal:  Panama.

(Note that two of these have repeated words; this is
disallowed by our problem specification.)

The length is measured in characters NOT including
spaces.  The input sets will have no repeated words.

The input will consist of multiple sets of words.  Each
word is on a single line by itself, and each set of
words ends with an empty line that contains nothing.
There are no space characters in any line.  Each word
consists solely of lowercase letters.

There will be fewer than 1000 words in all the input
word sets combined and no word will be longer than 80
characters.

Output is a single line containing the unique longest
palindrome made from each word set.  The input word sets
will be chosen so that there is exactly one longest
palindrome that can be made from the word set without
repeating words.  There must be a single space character
between consecutive words in the output line, and no
other space characters in the line.  The line may be
very long.

The words and the palindromes may not necessarily make
sense.

Sample input:

an
bann
nab
foo

code
coder
contest
fast
gnu
problem
redo
reverse
rung

Sample output:

bann an nab
gnu redo code rung