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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 leaves(tree):
 """ Return the leaves in this subtree. """

    lvs = []
    def _leaves(node):
        if np.isscalar(node):
            lvs.append(node)
        elif isinstance(node, tuple) and len(node) == 2:
            _leaves(node[0])
            _leaves(node[1])
        else:
            raise Exception("Not a tree!")

    _leaves(tree)
    return lvs