Function build 

pub fn build<F, O: Clone, A: HasVar>(f: F) -> BuildResult<O, A>
where
    F: Fn(&Rc<RefCell<OpenHypergraph<O, A>>>) -> (Vec<Var<O, A>>, Vec<Var<O, A>>),

Construct an OpenHypergraph from a function taking an empty OpenHypergraph, and returning two lists of Vars corresponding to sources and targets.