public SetAction set(IfNotExistsFunction<NS> ifNotExistsFunction) {
        return new SetAction(this, ifNotExistsFunction);
    }
Returns a <code>SetAction</code> object used for building update
expression. If the attribute referred to by this path operand doesn't
exist, the returned object represents adding the value of evaluating the
specified <code>IfNotExists</code> function as an attribute to an item.
If the attribute referred to by this path operand already exists, the
returned object represents the value replacement of the current attribute
by the value of evaluating the specified <code>IfNotExists</code>
function.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/xspec/NS.java