@Override
    protected void onDeserialize(Deserializer buf) {
        super.onDeserialize(buf);
        keyAttribute = getUtf8(buf);
        valueAttribute = getUtf8(buf);
        key = getUtf8(buf);
        keySourceAttribute = getUtf8(buf);
    }
This function is an instruction to do a lookup in a map attribute, returning the value.

The key is either specified explicitly or found via a key source attribute.
Two underlying attributes are used to represent the map attribute (the key and value attributes).

@author geirst
searchlib/src/main/java/com/yahoo/searchlib/expression/AttributeMapLookupNode.java