Function partial_trace_sector_density 

pub fn partial_trace_sector_density<State>(
    density: &[Complex64],
    basis_states: &[State],
    total_sites: usize,
    retained_sites: &[usize],
    noncommuting_groups: &[NoncommutingGroup],
) -> Result<Vec<Complex64>>
where
    State: BinaryState,

Reduced density matrix of a mixed state stored in an arbitrary binary sector basis.

Only pairs with the same environment key are visited. The input remains a dense matrix in the selected sector, but no unrestricted parent density matrix is formed.