/// Makes the user the owner of a position. The position must not have an owner.
fn grant_position(&mut self, owner: Address, id: U256) {
    // set owner
    self.position_owners.setter(id).set(owner);
    
    // increment count
    let owned_positions_count = self.owned_positions.get(owner) + U256::one();
    self.owned_positions
        .setter(owner)
        .set(owned_positions_count);
}
