Macro flow_base_let

macro_rules! flow_base_let {
    ($block:block ?$opt:expr) => { ... };
    ($block:block *$($opt:tt)*) => { ... };
    ($block:block $($var:ident)* =$opt:expr $(,$($plus:tt)*)*) => { ... };
    ($block:block ($($($var:ident)*),*)=$opt:expr $(,$($plus:tt)*)*) => { ... };
    ($block:block [$($($var:ident)*),*]=$opt:expr $(,$($plus:tt)*)*) => { ... };
}