PCFG unfold in Mathematica

gistfile1.txt

TerminalQ[x_] := MemberQ[{a, b, c, d}, x];

Rules = {S -> {{{X, Y}, 1}, {{X, X, Y}, 5}}, 
   X -> {{{a}, 1/2}, {{b}, 1/2}}, Y -> {{{c}, 1/3}, {{d}, 2/3}}};

PCFGUnfold[sym_, Rules_, TerminalQ_] :=
  If[TerminalQ[sym],
   {{{sym}, 1}},
   Block[{lookup, rules, probabilities},
    (*for some reason,I can't specify the last two in the {} above*)
 
       lookup = sym /. Rules;
    rules = #[[1]] & /@ lookup;
    probabilities = 
     Normalize[#[[2]] & /@ lookup, 
      Total];(*divide pcfg weights by their Total*)Join @@ MapThread[
      (*a production is a list whose first element is a sequence of \
symbols and whose second element is a probability*)
      
      Function[{production, prob},
       Block[(*subprods is a list of productions,
        one for each symbol in prod*){subProds = 
          PCFGUnfold[#, Rules, TerminalQ] & /@ production},
        Map[
         Function[prod,
          List[
           List @@ (First /@ prod),
           prob*Times @@ (#[[2]] & /@ prod)]],
         Tuples[subProds]]]],
      {rules, probabilities}]
    ]
   ];

PCFGUnfold[S, Rules, TerminalQ]