
Alias record (alias -> substitute)

* If the alias is already public, ignore the alias record.

* If the alias is already aliased, signal a warning.

* If the subst is not already public, save a back ptr from
  subst->alias.

* If the alias becomes public later, the alias is removed.



SYMFL_SUBST

Means that SYM_SUBST field is valid as the alias SYM_STR, and points
to the subst symbol


SYMFL_ALIAS

Is used to catch duplicate alias records.


