Either a vector of values for the numerators,
or, if denom is NULL,
fractions using "/" as the separator
(allows passing fractions directly, as from fractions)
denom
Vector of values for the denominators, or NULL (the default)
if num is already formatted fractions.
format
Character vector of length one giving the format of the output.
Default of "markdown" uses super script for the numerator,
a slash, then subscript for the denominator.
Alternatively, "latex" uses \frac and surrounds
the fraction with $ to induce LaTeX conversion
(via MathJax for html output)