code: public static Environment declareThisWithin(Decl.FunctionOrMethod decl, Environment environment) {
  		if (decl instanceof Decl.Method) {
  			Decl.Method method = (Decl.Method) decl;
  			environment = environment.declareWithin("this", method.getLifetimes());
  		}
  		return environment;
  	}
docs: Update the environment to reflect the fact that the special "this" lifetime
  is contained within all declared lifetime parameters. Observe that this only
  makes sense if the enclosing declaration is for a method.
  
  @param decl
  @param environment
  @return