public SetAction set(NS source) {
        return new SetAction(this, source);
    }
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 attribute value of the
specified source path operand to an item. If the current attribute
already exists, the returned object represents the value replacement of
the current attribute by the attribute value of the specified source path
operand.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/xspec/NS.java