openfermion.utils.up_then_down

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up then down reordering, given the operator has the default even-odd

openfermion.utils.up_then_down(
    mode_idx, num_modes
)

ordering. Otherwise this function will reorder indices where all even indices now come before odd indices.

Example: 0,1,2,3,4,5 -> 0,2,4,1,3,5

The function takes in the index of the mode that will be relabeled and the total number modes.

Returns (int): reordered index of the mode.