An invocation pp_tag t will produce a pretty representation for tag
t. Such a pretty-printer can be used to produce outputs, or return
strings, or to combine with other pretty representations to create
compound values.
FAILURE
Never fails.
EXAMPLE
> show_tags := true;
val it = () : unit
> Portable.pprint Tag.pp_tag (Tag.read "fooble");
[oracles: fooble] [axioms: ]
val it = (): unit