[R] dataframe

[n.vialma at libero.it]

Dear list,
I have a question about the selection of Variables in dataframes.
I have a dataframes like this:

CFISCA                       FIRMS                              
YEAR                    VAR               VALUE
20345                            nike                                 
2005                       EC01                 34
20345                            nike                                  
2006                      EC01                   45
56779                           mediaset                          
2005                       EC01                 65
64568                           agazzini                            
2005                       EC01                  78
64568                           agazzini                           
2006                        EC01                   56
78907                           uniteam                           
2006                         EC01                    46


what my result should be is a new dataframes which have only firms which have 
data in both years (2005,2006)

CFISCA                       FIRMS                              
YEAR                    VAR               VALUE
20345                            nike                                 
2005                       EC01                 34
20345                            nike                                  
2006                      EC01                   45
64568                           agazzini                            
2005                       EC01                  78
64568                           agazzini                           
2006                        EC01                   56


Thanks for your attention!!!