code: public String equationsToString(String prefix, int fractionDigits) {
      DecimalFormat nf = new DecimalFormat();
      nf.setMinimumFractionDigits(fractionDigits);
      nf.setMaximumFractionDigits(fractionDigits);
      nf.setDecimalFormatSymbols(new DecimalFormatSymbols(Locale.US));
      nf.setNegativePrefix("");
      nf.setPositivePrefix("");
      return equationsToString(prefix, nf);
    }
docs: Returns a string representation of this equation system.
  
  @param prefix the prefix of each line
  @param fractionDigits the number of fraction digits for output accuracy
  @return a string representation of this equation system