Function is_solvable

fn is_solvable(
    equation: &Equation,
    ops: &Vec<fn(_: i64, _: i64) -> Option<i64>>,
) -> bool

Do depth-first search to solve the equation using permutations of the available operators, returning true if there is at least one permutation of operators that results in the target number. Operations must increase the total, or return None for all possible inputs.