ignore_ops,
quoted, quote_non_ascii, brace_terms(false)
and
numbervars after numbervars/4
using the singletons option.
Note that due to the use of numbervars/4,
non-ground terms must be written using a single write_canonical/1
call. This used to be the case anyhow, as garbage collection between
multiple calls to one of the write predicates can change the _<NNN>
identity of the variables.