avoid `.clone()` where possible

it was only there to please the type system, now we use references in
the `is_clause_*` functions directly.

rust/solve/src/dpll.rs

}
/// A satisfied clause is a clause where at least one atom is true.
fn is_clause_satisfied(vars: &BoundVars, clause: Clause) -> bool {
fn is_clause_satisfied(vars: &BoundVars, clause: &Clause) -> bool {
    clause.iter().any(|v| is_true(vars, *v))
}
#[test]
fn test_is_clause_satisfied() {
    assert!(is_clause_satisfied(&from_vec(vec!(1)), vec!(1)));
    assert!(!is_clause_satisfied(&empty_vars(), vec!(1)));
    assert!(is_clause_satisfied(&from_vec(vec!(1)), &vec!(1)));
    assert!(!is_clause_satisfied(&empty_vars(), &vec!(1)));
}
/// A conflict clause is a clause whose atoms each are false.
fn is_clause_conflict(vars: &BoundVars, clause: Clause) -> bool {
fn is_clause_conflict(vars: &BoundVars, clause: &Clause) -> bool {
    clause.iter().all(|v| is_false(vars, *v))
}
#[test]
fn test_is_clause_conflict() {
    assert!(is_clause_conflict(&from_vec(vec!(-1)), vec!(1)));
    assert!(is_clause_conflict(&from_vec(vec!(-1, -2, -3)), vec!(1, 2, 3)));
    assert!(is_clause_conflict(&from_vec(vec!(-1, -2, -3, 4)), vec!(1, 2, 3)));
    assert!(!is_clause_conflict(&from_vec(vec!(1)), vec!(1)));
    assert!(!is_clause_conflict(&from_vec(vec!(2)), vec!(1, 2)));
    assert!(is_clause_conflict(&from_vec(vec!(-1)), &vec!(1)));
    assert!(is_clause_conflict(&from_vec(vec!(-1, -2, -3)), &vec!(1, 2, 3)));
    assert!(is_clause_conflict(&from_vec(vec!(-1, -2, -3, 4)), &vec!(1, 2, 3)));
    assert!(!is_clause_conflict(&from_vec(vec!(1)), &vec!(1)));
    assert!(!is_clause_conflict(&from_vec(vec!(2)), &vec!(1, 2)));
}
/// A unit clause is a clause where one atom is unknown and all
/// others are false.
fn is_clause_unit(vars: &BoundVars, clause: Clause) -> bool {
fn is_clause_unit(vars: &BoundVars, clause: &Clause) -> bool {
    let mut unknowns = 0;
    
    for v in clause {
    for &v in clause {
        if is_unknown(vars, v) {
            unknowns += 1
        }
#[test]
fn test_is_clause_unit() {
    let vars = &from_vec(vec!(1, 2));
    assert!(is_clause_unit(vars, vec!(3)));
    assert!(is_clause_unit(vars, vec!(-1, 3)));
    assert!(is_clause_unit(vars, vec!(-1, -2, 3)));
    assert!(!is_clause_unit(vars, vec!(1, 3)));
    assert!(!is_clause_unit(vars, vec!(1, 2, 3)));
    assert!(!is_clause_unit(vars, vec!(1, 2)));
    assert!(is_clause_unit(vars, &vec!(3)));
    assert!(is_clause_unit(vars, &vec!(-1, 3)));
    assert!(is_clause_unit(vars, &vec!(-1, -2, 3)));
    assert!(!is_clause_unit(vars, &vec!(1, 3)));
    assert!(!is_clause_unit(vars, &vec!(1, 2, 3)));
    assert!(!is_clause_unit(vars, &vec!(1, 2)));
}
pub fn dpll(clauses: Vec<Clause>) -> Option<BoundVars> {
    let mut vars = &mut empty_vars();
    
    loop {
        if clauses.iter().all(|c| is_clause_satisfied(&vars, c.clone())) { // all clauses satisfied, success
        if clauses.iter().all(|c| is_clause_satisfied(&vars, c)) { // all clauses satisfied, success
            //println!("satisfied");
            return Some(vars.clone())
        } else if clauses.iter().any(|c| is_clause_conflict(&vars, c.clone())) { // a conflict exists, backtrack
        } else if clauses.iter().any(|c| is_clause_conflict(&vars, c)) { // a conflict exists, backtrack
            match stack.pop() {
                None => return None, // nothing to backtrack, no solution found
                Some((v, b)) => {
                    vars.insert(v);
                }
            }
        } else if clauses.iter().any(|c| is_clause_unit(&vars, c.clone())) { // a unit clause exists, propagate
        } else if clauses.iter().any(|c| is_clause_unit(&vars, c)) { // a unit clause exists, propagate
            let cs = clauses.clone();
            let clause = cs.iter().find(|&c| is_clause_unit(&vars, c.clone())).unwrap();
            let clause = cs.iter().find(|&c| is_clause_unit(&vars, c)).unwrap();
            let unknown = *clause.iter().find(|&v| is_unknown(&vars, *v)).unwrap();
            //println!("propagate {} from {:?}", unknown, clause);
            vars.insert(unknown);