Keep the product in wishlist after the product added to the cart

In Drupal 8 you can do this out of the box. Put a reference field in the parent and target it on the child content type. The label of the field would be "In this section". You can theme the section in a field twig of this specific field, for example field--field-reference.html.twig.

You can nest the entities, but if you reference the same content type and you leave the reference field in the view mode of the child there is a risk of an infinite loop, which will result in a time-out.

In terms of a dielectric, it means there is a linear constitutive relation between the vectors.

$${\boldsymbol D} = \epsilon_0 {\boldsymbol E} + {\boldsymbol P}$$

Or

$${\boldsymbol D} = \epsilon_r \epsilon_0 {\boldsymbol E}$$

where $\epsilon_r \epsilon_0$ is a scalar relative and vacuum permitivitty. This way, there is now a linear relation between the applied field and the electric displacement. There are certainly situations where

$$D_j = \epsilon_{ij} E_i$$

for some rank two tensor $\epsilon_{ij}$, ie it is more favorable to apply a field along one direction and get a response along another -- re: anisotropy. If a dielectric material is free of saturation, then you can apply as much of a field as you want and get a corresponding electric displacement.

A hysteresis loop in a ferroelectric for example, does not allow this since there is a saturation for large field, so the mathematical way of expressing a dielectric material without a hysteresis is exactly the linear relation above for the electric displacement(and corresponding polarization). From Wiki:

In your first quote here, the author says the material is free of sources, which includes the bound charge source $\rho_b$ that arises $-\nabla \cdot{\boldsymbol P} = \rho_b \Rightarrow {\boldsymbol P} = 0$ as in the above comment. Think of a dielectric sphere in an external field problem that can be solved with separation of variables. There exists $\mathbf{no}$ bound charges within the material so the field inside is aligned with the external field and linearly increases with it.

You are also right that no polarization or magnetization should remain when the applied fields are removed $\mathbf{if}$ the material is free of hysteresis (see above hysteresis plot for zero ${\boldsymbol E} \Rightarrow {\boldsymbol D} \neq 0 $). Please let me know if I can clarify or expand anything else in the comments.