Macro alt 

macro_rules! alt {
    () => { ... };
    ($parser:expr $(,)?) => { ... };
    ($parser_head:expr, $($parser_tail:expr),+ $(,)?) => { ... };
    (@attempt $stream:ident $last:expr) => { ... };
    (@attempt $stream:ident $parser_curr:expr, $($parser_rest:expr),+) => { ... };
}

Chain together multiple alternative parsers using the binary alt combinator.