<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_coo(self, fp=None, vartype_header=False):
 """Serialize the binary quadratic model to a COOrdinate_ format encoding. .. _COOrdinate: https://en.wikipedia.org/wiki/Sparse_matrix#Coordinate_list_(COO) Args: fp (file, optional):
 `.write()`-supporting `file object`_ to save the linear and quadratic biases of a binary quadratic model to. The model is stored as a list of 3-tuples, (i, j, bias), where :math:`i=j` for linear biases. If not provided, returns a string. vartype_header (bool, optional, default=False):
 If true, the binary quadratic model's variable type as prepended to the string or file as a header. .. _file object: https://docs.python.org/3/glossary.html#term-file-object .. note:: Variables must use index lables (numeric lables). Binary quadratic models saved to COOrdinate format encoding do not preserve offsets. Examples: This is an example of a binary quadratic model encoded in COOrdinate format. .. code-block:: none 0 0 0.50000 0 1 0.50000 1 1 -1.50000 The Coordinate format with a header .. code-block:: none # vartype=SPIN 0 0 0.50000 0 1 0.50000 1 1 -1.50000 This is an example of writing a binary quadratic model to a COOrdinate-format file. This is an example of writing a binary quadratic model to a COOrdinate-format string. 0 0 -1.000000 0 1 -1.000000 1 1 1.000000 """

        import dimod.serialization.coo as coo

        if fp is None:
            return coo.dumps(self, vartype_header)
        else:
            coo.dump(self, fp, vartype_header)