code: @Override public void init(boolean expensive) {
      super.init(expensive);
      for( double p : _parms._probs )
        if( p < 0.0 || p > 1.0 )
          error("_probs","Probabilities must be between 0 and 1");
      _ncols = train().numCols()-numSpecialCols(); //offset/weights/nfold - should only ever be weights
      if ( numSpecialCols() == 1 && _weights == null)
        throw new IllegalArgumentException("The only special Vec that is supported for Quantiles is observation weights.");
      if ( numSpecialCols() >1 ) throw new IllegalArgumentException("Cannot handle more than 1 special vec (weights)");
    }
docs: Initialize the ModelBuilder, validating all arguments and preparing the
  training frame.  This call is expected to be overridden in the subclasses
  and each subclass will start with "super.init();".  This call is made
  by the front-end whenever the GUI is clicked, and needs to be fast;
  heavy-weight prep needs to wait for the trainModel() call.
  
  Validate the probs.