A call to pprint ppf x will call the pretty-printing function
ppf on value x, with the pretty-printing output printed.
string that is eventually returned to the user. The linewidth used for
determining when to wrap with newline characters is 72.
FAILURE
Fails if the pretty-printing function fails on the particular input
value.
EXAMPLE
> pprint PP.add_string "hello";
hello
val it = (): unit