[R] Data frame aggregation

[PHILIP GLADWIN, BLOOMBERG TRADEBOOK:]

Hello,
Could anybody help me with this question?

Example data frame
NAME     TICKER     SHARES     PERFORMANCE
John      ABC        100        0.05       
John      ABC        1000       1.5        
Alice     EFG        20         0.3       
Paul      HIJ        50         1.0
Paul      JKL        60         2.0
Paul      MNO        12         3.0


I would like to aggregate this dataframe by NAME, TICKER, and SHARES and to 
perform a weighted mean.  I expect the output to be
NAME   TICKER     SHARES       PERFORMANCE
John   ABC        1000+100     ( (100 * 0.05) + (1000 * 1.5) ) / (100 + 1000)
Alice  EFG        20           0.3
Paul   HIJ        50           1.0
Paul   JKL        60           2.0
Paul   MNO        12           3.0

and I would like the output to be in a data frame.

Can anybody help?
Thank you,
Phil,

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