ensure all functions are internal

[geoknee]

This means the library is an Embedded library, not a Linked library

https://medium.com/coinmonks/all-you-should-know-about-libraries-in-solidity-dd8bc953eae7

[geoknee]

We might be able to write a test that fails if we accidentally went back down the linked library route. For example, we could parse the transaction output of one of our tests and assert that there are no DELEGATECALL opcodes.

[geoknee]

...or we could ensure that our test suite records the gas consumed down enough execution pathways to detect the increased gas usage of DELEGATECALL. I think that is not currently the case because we only use the types, not the functions, exposed by the library.

Either strategy requires greater test coverage.

[NiloCK]

Hooray for deployable contracts.

Notable here is that WithdrawHelper and ERC20Interface still declare external functions and are consumed by the library. I understand this to be fine because they do not describe implementations that a consumer of exit-format would be required to run.