Function types -
https://docs.soliditylang.org/en/v0.5.3/structure-of-a-contract.html?highlight=function#functions https://docs.soliditylang.org/en/v0.5.3/contracts.html?highlight=fallback#fallback-function https://docs.soliditylang.org/en/v0.5.3/contracts.html#function-modifiers
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