This function should be called no more than once in a code chunk where
'figure' was set to true. Legal values for numformat are 'A', 'a', 'I', 'i', and '1'.
If numformat is 'A' or 'a', numlettersync specifies what style of numbering
to use after the first 26 figures. If numlettersync is true, the next 26
figures will be numbered 'AA', 'BB', ..., 'ZZ', 'AAA', 'BBB', etc.
If numlettersync is false, the subsequent figures will be numbered 'AA', 'AB',
..., 'AZ', 'BA', 'BB', ..., 'BZ', etc.
The default formula, which numbers figures consecutively, is usually desired,
but you could specify a formula of 'Illustration+10' to have your figures
numbered 1, 11, 21, etc.