Events:

#[contracttype]
#[derive(Clone, Event)]
pub struct Transfer {
    #[topic]
    pub from: Address,
    #[topic]
    pub to: Address,
    #[topic]
    pub token: Address,
    pub value: u128,
}

pub fn transfer(env: Env, from: Address, to: Address, token: Address, value: u128) {
    Transfer {
        from,
        to,
        token,
        value,
    }
    .publish(&env);
}