Macro action

macro_rules! action {
    ((let $name:ident $value:tt)) => { ... };
    ((set ($f:ident $($x:tt)*) $value:tt)) => { ... };
    ((delete ($f:ident $($x:tt)*))) => { ... };
    ((subsume ($f:ident $($x:tt)*))) => { ... };
    ((union $x:tt $y:tt)) => { ... };
    ((panic $message:literal)) => { ... };
    ($x:tt) => { ... };
}