docs: public String equationsToString(String prefix, NumberFormat nf) {
      if((coeff == null) || (rhs == null) || (row == null) || (col == null)) {
        throw new NullPointerException();
      }
  
      int[] coeffDigits = maxIntegerDigits(coeff);
      int rhsDigits = maxIntegerDigits(rhs);
  
      StringBuilder buffer = new StringBuilder();
      buffer.append(prefix).append('\n').append(prefix);
      for(int i = 0; i < coeff.length; i++) {
        for(int j = 0; j < coeff[row[0]].length; j++) {
          format(nf, buffer, coeff[row[i]][col[j]], coeffDigits[col[j]]);
          buffer.append(" * x_").append(col[j]);
        }
        buffer.append(" =");
        format(nf, buffer, rhs[row[i]], rhsDigits);
  
        if(i < coeff.length - 1) {
          buffer.append('\n').append(prefix);
        }
        else {
          buffer.append('\n').append(prefix);
        }
      }
      return buffer.toString();
    }