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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 build_tree(self, *args, **kwargs):
 """Dispatch a tree build call. Note that you need at least four taxa to express some evolutionary history on an unrooted tree."""

        # Check length #
        assert len(self) > 3
        # Default option #
        algorithm = kwargs.pop(kwargs, None)
        if algorithm is None: algorithm = 'raxml'
        # Dispatch #
        if algorithm is 'raxml':    return self.build_tree_raxml(*args, **kwargs)
        if algorithm is 'fasttree': return self.build_tree_fast(*args, **kwargs)