var pcfgTransition = function(symbol) {
  var rules = {
    'start': {rhs: [['NP', 'V', 'NP'], ['NP', 'V']], probs: [0.4, 0.6]},
    'NP': {rhs: [['A', 'NP'], ['N']], probs: [0.4, 0.6]}
  };
  return rules[symbol].rhs[discrete(rules[symbol].probs)]
}

var preTerminal = function(symbol) {
  return symbol == 'N' || symbol == 'V' || symbol == 'A'
}

var terminal = function(symbol) {
  var rules = {
    'N': {words: ['John', 'soup'], probs: [0.6, 0.4]},
    'V': {words: ['loves', 'hates', 'runs'], probs: [0.3, 0.3, 0.4]},
    'A': {words: ['tall', 'salty'], probs: [0.6, 0.4]} }
  return rules[symbol].words[discrete(rules[symbol].probs)]
}

var pcfg = function(symbol, yieldsofar, trueyield) {
  if (preTerminal(symbol)) {
    var t = terminal(symbol)
    if (yieldsofar.length < trueyield.length) {
      factor(t == trueyield[yieldsofar.length] ? 0 : -Infinity)
    }
    return yieldsofar.concat([t])
  } else {
    return expand(pcfgTransition(symbol), yieldsofar, trueyield) }
}

var expand = function(symbols, yieldsofar, trueyield) {
  if (symbols.length == 0) {
    return yieldsofar;
  } else {
    return expand(
        symbols.slice(1),
        pcfg(symbols[0], yieldsofar, trueyield),
        trueyield);
  }
}

Enumerate(function() {
  var y = pcfg('start', [], ['tall', 'John'])
  return y[2] ? y[2] : '' //distribution on next word?
}, 300)