A supplier and generator-on-demand of document Page objects.
This acts like a list of Page objects which generates them as needed. Externally, it can be used mostly as a list. If an index is requested for which no Page yet exists, that page will be generated, as well as any missing pages between the previous last page and the one requested. Consequently, keep in mind that innocent looking operations such as some_page_suppler are actually expensive operations, as they implicitly generate thousands of Page objects.
The contents of the PageSupplier should be treated as immutable. Attempts to modify the pages it contains will likely result in unexpected behavior.
This is an internal class meant to be created by the global Document
(Document): The global document using this object.