public ValueMap(PType <T> itemType) => ItemType = itemType;
public ValueList(PType <T> itemType) => ItemType = itemType;
/// <summary>
/// Initializes a new instance of <see cref="Derived{TBase, T}"/> with a base <see cref="PType{TBase}"/>.
/// </summary>
/// <param name="baseType">
/// The base <see cref="PType{TBase}"/> to depend on.
/// </param>
/// <exception cref="ArgumentNullException">
/// <paramref name="baseType"/> is null.
/// </exception>
protected Derived(PType <TBase> baseType)
=> BaseType = baseType ?? throw new ArgumentNullException(nameof(baseType));