If non-nil, add position of read symbols to `read-symbol-positions-list'.
If this variable is a buffer, then only forms read from that buffer
will be added to `read-symbol-positions-list'.
If this variable is t, then all read forms will be added.
The effect of all other values other than nil are not currently
defined, although they may be in the future.
The positions are relative to the last call to `read' or
`read-from-string'. It is probably a bad idea to set this variable at
the toplevel; bind it instead.