class_docstr: Simple matrix data type.
  
  Of course there are much more advanced matrix data types for Python (for
  instance, the C{ndarray} data type of Numeric Python) and this implementation
  does not want to compete with them. The only role of this data type is to
  provide a convenient interface for the matrices returned by the C{Graph}
  object (for instance, allow indexing with tuples in the case of adjacency
  matrices and so on).
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code: def __str__(self):
      rows = (
          "[%s]" % ", ".join(repr(item) for item in row) for row in self
      )
      return "[%s]" % "\n ".join(rows)
  
label_desc: Use image node_id 18 for calling the Matrix obj's underlying member method code with example usage: obj.__str__() and returns: unknown