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#! /usr/bin/perl

...
  printf "%-4s: %20s,  %-8s %6s\n",
      $_->[0], qq($_->[0]<-$_->[3]), $_->[2], $group{$_->[2]};
}

M19,Q,P,
M31,M19,Pl,
...
M549,M73,E,
M550,M73,E,
M73,Q,P,

M19 :               M19<-Q,  P           I_1
M73 :               M73<-Q,  P           I_1
...
M33 :     M33<-M31<-M19<-Q,  P.Pl.E    II_19
M420:    M420<-M31<-M19<-Q,  P.Pl.E    II_19
M421:    M421<-M31<-M19<-Q,  P.Pl.E    II_19