Ref Key

Key Ref Key of function Price in Tradable refers to the key associated with the parameter of which the fair value is requested.
It must be one of the keys contained in the object specified in Ref Object

This entry may even specify the key contained in a sub-object, which sub-object is contained in the specified object.
This is possible when the entry here is an array of keys that provide the inclusion path to the final key in the following fashion:
The first key is used to identify a sub-object that lives inside the original specified object.
The second key identifies a sub-object that lives inside the sub-object associated with the first key.
This identification of sub-objects continues until we reach the last key in the array.
This last key must be associated with a numerical value, which will be perturbed by the root finder solver until it equals the returned fair value that fulfils the desired criteria.