Macro funfsm::precondition

macro_rules! precondition {
    ($c:ident, $s:expr, $p:expr) => { ... };
}

Take a constraints object ($c), the &'static str name of the state ($s), and a predicate closure ($p). Box the closure and store it into the preconditions hashmap under it's state name along with an associated error message to use if the predicate fails.