if (m_attribute == null) { return "No model built yet."; }
  public String toString() {    if (m_attribute == null) {      return "No model built yet.";    }    StringBuffer text = new StringBuffer();    if (m_attribute == null) {      text.append("Predicting constant " + m_intercept);    } else {      text.append("Linear regression on " + m_attribute.name() + "\n\n");      text.append(Utils.doubleToString(m_slope,2) + " * " + 		m_attribute.name());      if (m_intercept > 0) {	text.append(" + " + Utils.doubleToString(m_intercept, 2));      } else {      text.append(" - " + Utils.doubleToString((-m_intercept), 2));       }    }    text.append("\n");    return text.toString();  }