my %CodonMap = ('GCA'=>'A', 'GCC'=>'A', 'GCG'=>'A', 'GCU'=>'A');
my @acodes = ();
for my $key (keys %CodonMap) {
    next unless $CodonMap{$key} eq 'A';
    push @acodes, $key;
}
print join ("\n", @acodes, '');