Function cross_voltage_coefficients 

pub fn cross_voltage_coefficients(
    reference_phi: Complex64,
    reference_psi: Complex64,
) -> Result<CrossVoltageCoefficients>

Form the fixed-angle first-order closure of v_phi * conj(v_psi).

At the reference point it reproduces the product exactly. Away from that point it is the first-order Taylor expansion of sqrt(w_phi w_psi) exp(j delta_theta) with the angle difference fixed.

Errors

Either reference phasor is zero or non-finite.