The OnEmpty class represents an action to be taken by the bytecode generator when compiling an expression
to return a "primitive" (unboxed) value such as a double float int long or String (treated as primitive for this
purpose). There are a number of defined subclasses and these are known to users of the class; it is not possible
to introduce new subclasses without careful examination of existing code.
The UnwindAndJump subclass causes the code generator, when an expression evaluates to the empty sequence,
to generate a "goto" instruction to a defined label; but before executing the jump, the stack will be unwound
to a known position.