A named list containing two data.frames, one
for calls and one for puts. If more than one
expiration was requested, this two-element list
will be contained within list of length length(Exp).
Each element of this list will be named with the expiration
month and year (for Yahoo sourced data).
If Exp is set to NULL, all expirations
will be returned.
Details
This function is a wrapper to data-provider specific
APIs. By default the data is sourced from yahoo.