fn differential_add_and_double(
P: &mut ProjectiveMontgomeryPoint,
Q: &mut ProjectiveMontgomeryPoint,
affine_PmQ: &FieldElement,
)fn differential_add_and_double(
P: &mut ProjectiveMontgomeryPoint,
Q: &mut ProjectiveMontgomeryPoint,
affine_PmQ: &FieldElement,
)