Macro defrel 

macro_rules! defrel {
    ($(#[$outer:meta])* pub $name:ident($($args:ident),*) { $($g:expr),* $(,)? }) => { ... };
    ($(#[$outer:meta])* $name:ident($($args:ident),*) { $($g:expr),* $(,)? }) => { ... };
    ($(#[$outer:meta])* pub trace $name:ident($($args:ident),*) { $($g:expr),* $(,)? }) => { ... };
    ($(#[$outer:meta])* trace $name:ident($($args:ident),*) { $($g:expr),* $(,)? }) => { ... };
    (pub $name:ident($($args:ident),*) { $($g:expr);* $(;)? }) => { ... };
    ($name:ident($($args:ident),*) { $($g:expr);* $(;)? }) => { ... };
    (@body: $($args:ident),* { $($g:expr),* }) => { ... };
    (@tracebody: $name:ident, $($args:ident),* { $($g:expr),* }) => { ... };
}

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