Functions must be annotated as `stateful` in order to
- Update the contract state (using `put`)
- Call `Chain.spend` or other primitive functions that cost tokens
- Call an Oracle or AENS function that requires a signature
- Make a remote call with a non-zero value
- Construct a lambda calling a stateful function
It does not need to be stateful to
- Read the contract state
- Call another contract with value=0, even when the remote function is stateful
