my %rules = (
     a => [ 'b', 'c' ],
     b => [ 'c', 'd' ],
     c => [ 'd' ],
     e => [ 'b', 'a' ],
     f => undef,
);

##</code><code>##

my %rules = (
     a => [ 'b' ],
     b => [ 'c' ],
     c => [ 'd' ],
     e => [ 'a' ],
     f => undef,
);

##</code><code>##

my %rules = (
     a => [ 'b' ],
     b => [ 'c' ],
     c => [ 'd' ],
     e => [ 'b' ],
     f => undef,
);