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 my @probs =(0,0.1,0.53,0.51,0.59,0.67,0.2,0.04,0.05,0.56,0.89,0.75);
 my $t;
 print $_ . (",","\n")[$t=1-$t]  
     for  grep {($probs[$_] >=.5 .. $probs[$_]> .5) and $t=!$t }
            0..$#probs

2,4
9,11