How to verify messages that were signed by Smart Contract Account?

[Confucian-e]

I read verifyMessage chapter.
It says:

This Action supports verifying messages that were signed by either a Smart Contract Account or Externally Owned Account.

I know EOA can sign messages because it has private key. And Smart Contract can verify by using ecrecover function.
But how to sign messages by Smart Contract Account, and how to verify signature using solidity?