pub fn swap(\n    env: Env,\n    token_in: Address,\n    token_out: Address,\n    amount_in: i128,\n    amount_out_min: i128,\n) -> i128 {\n    let pair = get_pair(&env, &token_in, &token_out);\n    let (reserve_in, reserve_out) = pair.get_reserves();\n    let amount_out = get_amount_out(amount_in, reserve_in, reserve_out);\n    require!(amount_out >= amount_out_min, "insufficient output");\n    // ... transfer logic\n    amount_out\n}