FTStateTargeted replaces the standard
tgt,sel mechanism with symbolic variable access. Argument lists are the
same as in Fox, but with the tgt, sel arguments
replaced with a target object (or proc/method returning an object), a state
variable in the target, and possibly additional args.
Class which include this module must define an instance method target_arg_range which returns
the range of positions in the argument list which take the place of
state must be name of observable variable in target