Macro defer 

macro_rules! defer {
    (return $val:expr) => { ... };
    (return) => { ... };
    (break) => { ... };
    (continue) => { ... };
    ($val:expr) => { ... };
    () => { ... };
}

Create a deferred control-flow command.

This takes a command (e.g. return value, break, continue, etc.) and creates the command in the form of the Command enum. This is deferred (that is, it is not runned instantly) until one executes the Command, which is done through run!() and run_loop!() depending on whether or not you are in a loop.