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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_statements(self, pmid, stmts):
 """Add INDRA Statements to the incremental model indexed by PMID. Parameters pmid : str The PMID of the paper from which statements were extracted. stmts : list[indra.statements.Statement] A list of INDRA Statements to be added to the model. """

        if pmid not in self.stmts:
            self.stmts[pmid] = stmts
        else:
            self.stmts[pmid] += stmts