Availability:built-in
ignore_ops,
quoted 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 _G<NNN> 
identity of the variables.