E0051: yield outside main fn

Coroutine yield expressions may only appear inside coroutines functions, specifically main fns inside utxos.

Example (error)

fn example() {
    yield();
}

Fix

Use separate if ... is blocks for each method call:

abi ExampleAbi { }
utxo Example {
    impl ExampleAbi { }
    main fn example() {
        yield(ExampleAbi);
    }
}