public Builder output(ToShapeId outputShape) {
            output = outputShape == null ? UnitTypeTrait.UNIT : outputShape.toShapeId();
            return this;
        }
Sets the output shape ID of the operation.

@param outputShape Shape ID that MUST reference a structure.
@return Returns the builder.
smithy-model/src/main/java/software/amazon/smithy/model/shapes/OperationShape.java