28. Internal helper functions

Since Vyper has no inheritance, code reuse within a contract happens through `@internal` helper functions rather than base contracts. This keeps every contract self-contained and readable without following a chain of `is` relationships.

@internal
def _safe_transfer(token: address, to: address, amount: uint256):
    success: bool = ERC20(token).transfer(to, amount)
    assert success, "transfer failed"

@external
def payout(token: address, to: address, amount: uint256):
    self._safe_transfer(token, to, amount)

Internal functions can call other internal functions, take any type Vyper supports, and are inlined-away at the bytecode level with no runtime dispatch overhead compared to an external call.

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