Example of wrong calculation of f90, due to the unsorted
towers of a "merged jet". This example is  taken from a jet 
resulting from the merge of 2 jets (jet taken at random, 
in a qcd sample):

Tower list (should be sorted according to decreasing p_T)
tower pT = 2.29183 ;  Eta = -0.340722   ;  Phi = 6.23063
tower pT = 1.46176 ;  Eta = -0.30055   ;  Phi = 6.13255
tower pT = 1.20526 ;  Eta = -0.251738   ;  Phi = 0.343836
tower pT = 1.14223 ;  Eta = -0.714646   ;  Phi = 0.0488658
tower pT = 1.09027 ;  Eta = 0.123796   ;  Phi = 6.13157
tower pT = 0.739867 ;  Eta = -0.232115   ;  Phi = 6.13225
tower pT = 0.640703 ;  Eta = 0.0225371   ;  Phi = 0.142381
tower pT = 0.611423 ;  Eta = -0.190565   ;  Phi = 0.340815
tower pT = 0.567447 ;  Eta = 0.022882   ;  Phi = 0.334618
tower pT = 0.419395 ;  Eta = -0.221091   ;  Phi = 6.23046
tower pT = 0.363401 ;  Eta = -0.124266   ;  Phi = 6.03395
tower pT = 0.294316 ;  Eta = -0.229832   ;  Phi = 6.03395
tower pT = 0.23374 ;  Eta = -0.337659   ;  Phi = 0.045881
tower pT = 0.20478 ;  Eta = -0.562479   ;  Phi = 0.242758
tower pT = 0.202289 ;  Eta = -0.412285   ;  Phi = 0.340815
tower pT = 0.100695 ;  Eta = 0.0158946   ;  Phi = 6.13
tower pT = 0.072931 ;  Eta = -0.0923428   ;  Phi = 5.93319
tower pT = 0.0584039 ;  Eta = -0.437648   ;  Phi = 0.339805
tower pT = 0.0446741 ;  Eta = 0.221687   ;  Phi = 0.142466
tower pT = 0.0363555 ;  Eta = 0.120793   ;  Phi = 0.339805
 here after is the merged jet: those towers should appear higher on 
     the list if the sorting would have been done after merging.
tower pT = 1.16905 ;  Eta = -0.189882   ;  Phi = 5.73836
tower pT = 0.172521 ;  Eta = 0.128609   ;  Phi = 5.83472
tower pT = 0.115442 ;  Eta = -0.248269   ;  Phi = 5.73742

    Split Merge work 1     ; nbmerge : 1    ; nbsplit : 0
    n90 : 21        ;  nItems : 23          ; 
    f90 : 0.913

when sorting correctly these towers, the resulting (correct)
    f90 is  0.61