@Override
    String asSubstituted(SubstitutionContext context) {
        return new StringBuilder(getPathOperand().asSubstituted(context))
                .append(" = ").append(getValue().asSubstituted(context))
                .toString();
    }
Returns the operand for this update expression as a string, substituted
if necessary with tokens using the given substitution context.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/xspec/SetAction.java