The format descriptor string used to print floats.
This is a %-spec like those accepted by `printf' in C,
but with some restrictions. It must start with the two characters `%.'.
After that comes an integer precision specification,
and then a letter which controls the format.
The letters allowed are `e', `f' and `g'.
Use `e' for exponential notation "DIG.DIGITSeEXPT"
Use `f' for decimal point notation "DIGITS.DIGITS".
Use `g' to choose the shorter of those two formats for the number at hand.
The precision in any of these cases is the number of digits following
the decimal point. With `f', a precision of 0 means to omit the
decimal point. 0 is not allowed with `e' or `g'.
A value of nil means to use the shortest notation
that represents the number without losing information.