the contract's balance is the cistern. it is not a separate account and it is not a mapping. it is one word of storage that the contract keeps equal to its own balance at all times, and the invariants page states that equality as the first thing to check.

there are two inflows and one outflow. curve entry and surcharge come in on mint. burn pay goes out on burn, and forty basis points of it never leaves. the contract has no receive or fallback that accepts eth outside of mint, so a plain transfer to its address reverts.