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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 true_events(network, previous_state, current_state, next_state, indices=None, major_complex=None):
 """Return all mechanisms that have true causes and true effects within the complex. Args: network (Network):
 The network to analyze. previous_state (tuple[int]):
 The state of the network at ``t - 1``. current_state (tuple[int]):
 The state of the network at ``t``. next_state (tuple[int]):
 The state of the network at ``t + 1``. Keyword Args: indices (tuple[int]):
 The indices of the major complex. major_complex (AcSystemIrreducibilityAnalysis):
 The major complex. If ``major_complex`` is given then ``indices`` is ignored. Returns: tuple[Event]: List of true events in the major complex. """

    # TODO: validate triplet of states

    if major_complex:
        nodes = major_complex.subsystem.node_indices
    elif indices:
        nodes = indices
    else:
        major_complex = compute.major_complex(network, current_state)
        nodes = major_complex.subsystem.node_indices

    return events(network, previous_state, current_state, next_state, nodes)