Macro defmatch 

macro_rules! defmatch {
    ($(#[$outer:meta])* pub $name:ident($($args:ident),*) { $($body:tt)* }) => { ... };
    ($(#[$outer:meta])* $name:ident($($args:ident),*) { $($body:tt)* }) => { ... };
}

Define a relation. A relation is a function that creates a goal.