@Override
    String asSubstituted(SubstitutionContext context) {
        StringBuilder sb = new StringBuilder(functionId).append("(").append(
                pathOperand.asSubstituted(context));
        if (operand != null)
            sb.append(", ").append(operand.asSubstituted(context));
        return sb.append(")").toString();
    }
Represents a <a href=
"http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.SpecifyingConditions.html#ConditionExpressionReference"
>Function</a> condition in building condition expression.
<p>
Underlying grammar:

<pre>
function ::=
   attribute_exists (path)
 | attribute_not_exists (path)
 | begins_with (path, operand)
 | contains (path, operand)
</pre>
<p>
This object is as immutable (or unmodifiable) as the underlying operand.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/xspec/FunctionCondition.java