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Applied on fields of ink! event types to indicate that they are topics.

Tells the ink! codegen to provide a topic hash for the given field. Every ink! event can only have a limited number of such topic field. The semnatics are similar to indexed event arguments in Solidity.


#[ink(event)]pub struct Transferred {    #[ink(topic)]    from: Option<AccountId>,
    #[ink(topic)]    to: Option<AccountId>,
    amount: Balance}