@Override
    String asSubstituted(SubstitutionContext context) {
        return "if_not_exists(" + attr.asSubstituted(context) + ","
                + operand.asSubstituted(context) + ")";
    }
Represents an <a href=
"http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.Modifying.html"
>if_not_exists(path, operand)</a> function in building expressions. If the
item does not contain an attribute at the specified path, then if_not_exists
evaluates to operand; otherwise, it evaluates to path. You can use this
function to avoid overwriting an attribute already present in the item.
<p>
This object is as immutable (or unmodifiable) as the underlying operand given
during construction.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/xspec/IfNotExistsFunction.java