A list mapping read symbols to their positions.
This variable is modified during calls to `read' or
`read-from-string', but only when `read-with-symbol-positions' is
Each element of the list looks like (SYMBOL . CHAR-POSITION), where
CHAR-POSITION is an integer giving the offset of that occurrence of the
symbol from the position where `read' or `read-from-string' started.
Note that a symbol will appear multiple times in this list, if it was
read multiple times. The list is in the same order as the symbols were read in.