Method return_value

sources/system/unix/common.cpp:372–378

			}

			virtual var return_value() const
			{
				if (state == fiber_state::finished)
					return ret_val;
				else
					throw lang_error("Fiber has not yet started or ended.");
			}
		};

		fiber_t create(const context_t &cxt, std::function<var()> f)