An entity that supports read and write access to a value.
Implementations include local variables, fields, and elements
of a list.
You can use this type to implement pass-by-reference semantics,
such as the assignment operation.
Corresponds to "lvalue" in C.
C++ supports pass-by-reference, but not the creation of
fully customizable reference entities.
There is no Java equivalent of this type, except for builtin
operators such as assignment and getter/setter convention for fields.