R help - Mar 2014 - using data.table ,plyr

I have a data A which looks like


    author_id paper_id prob
       731    24943    1
       731    24943    1
       731   688974    1
       731   964345    .8
       731  1201905    .9
       731  1267992    1
       736    249      .2
       736   6889      1
       736   94345    .7
       736  1201905    .9
       736  126992    .8

The output I am desiring is:

    author_id    paper_id
      731        24943,24943,688974,1201905,964345
      736        6889,1201945,126992,94345,249

That is paper_id are arranged according to decreasing order of
probability.How could this be done?

Thanks

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