@Override
    protected void onDeserialize(Deserializer buf) {
        super.onDeserialize(buf);
        value = (ResultNode)deserializeOptional(buf);
    }
This abstract expression node represents a function to execute.

@author baldersheim
@author Simon Thoresen Hult
searchlib/src/main/java/com/yahoo/searchlib/expression/ConstantNode.java