public Optional<StructureShape> getOutputShape(ToShapeId operation) {
        return Optional.ofNullable(outputs.get(operation.toShapeId()));
    }
Gets the optional output structure of an operation.

<p>Operations in the model always have output. This operation will
only return an empty optional if the given operation shape cannot
be found in the model or if it is not an operation shape.

@param operation Operation to get the output structure of.
@return Returns the optional operation output structure.
smithy-model/src/main/java/software/amazon/smithy/model/knowledge/OperationIndex.java