Logical expression of InMemoryDBFilterFields.
InMemoryDBFilterExpressions can be combined using the & and | operators to create
complex logical expressions that evaluate to the InMemoryDB Query language.
This presents an interface by which users can create complex queries
without having to know the InMemoryDB Query language.
Filter expressions are not initialized directly. Instead they are built
by combining InMemoryDBFilterFields using the & and | operators.