Example of an INDEX definition without BASE file in the ECL reference manual

The ECL reference manual states that the 'baserecordset' is optional for an INDEX definition.

baserecset Optional. The set of data records for which the index file has been created. If omitted,
all fields in the keysand payloadparameters must be fully qualified.

No example is given. I am in the process of divorcing a keyfile definition (to be used by Roxie) from the THOR keybuild definition, but I'm unable to create a INDEX definition without specifying a BASE file. 

An example would help me, and others I expect, no end.