code: public String mkString(String rowsDelimiter, String columnsDelimiter) {
          return mkString(DEFAULT_FORMATTER, rowsDelimiter, columnsDelimiter);
      }
docs: Converts this matrix into the string representation.
  
  @param rowsDelimiter the rows' delimiter
  @param columnsDelimiter the columns' delimiter
  
  @return the matrix converted to a string