8. Every function needs a visibility decorator

Unlike Solidity, where forgetting a visibility keyword silently defaults to `public`, Vyper simply refuses to compile a function without `@external` or `@internal`. This single rule has eliminated an entire historical class of Solidity bugs (accidentally public functions).

@external
def withdraw(amount: uint256):
    assert msg.sender == self.owner, "not owner"
    send(self.owner, amount)

You can stack multiple decorators on one function, e.g. `@external` with `@view`, `@payable`, or `@nonreentrant("lock")`, but the visibility decorator is always mandatory.

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