sclass WeightedProduction {
  S a, b, c; // a + b = c; for subclassing, b = null
  double weight;
  S simplificationHint, plausibilityFunction;
  
  *() {}
  *(S *a, S *b, S *c, double *weight) {}
  *(S *a, S *c, double *weight) {}
  
  toString {
    ret a + (b == null ? "" : " + " + b) + " = " + c
      + appendSquareBracketStuff(simplificationHint);
  }
}