public static AttributeMapLookupNode fromKey(String attributeExpression, String keyAttribute, String valueAttribute, String key) {
        return new AttributeMapLookupNode(attributeExpression, keyAttribute, valueAttribute, key, "");
    }
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