* Test SLIF action names
  * When shadowed by CODE slot
  * Scalar but not a ref -- should fatal
  * !Scalar and !Code slot -- should fail to resolve
  * ref to code, scalar, hash, array -- should resolve as per NAIF

* Document
  * fully qualified actions now allowed in SLIF
  * null_ranking adverb added
  * $slr->series_restart().

* Display for Null POD

Later:

* Add pseudo-symbols: :null, :empty -- sugar for empty RHS alternative

* Add pseudo-symbols: :external, :never -- sugar for [^\d\D]

* Problem with indirect nulling.  Modifying Semantics::Null to add explicit
  empty production changes semantics.  "Topmost nulled" rule seems not to be
  followed in this case.  This is a "later to do" because the same issue exists
  in the NAIF.

* Test SLIF action names
  * When shadowed by CODE slot
  * Scalar but not a ref -- should fatal
  * !Scalar and !Code slot -- should fail to resolve
  * ref to code, scalar, hash, array -- should resolve as per NAIF

* Document
  * fully qualified actions now allowed in SLIF
  * null_ranking adverb added
  * $slg->set, $slr_set for tested named arguments
  * $slr->series_restart().

* Display for Null POD

Later:

* Add pseudo-symbols: :null, :empty -- sugar for empty RHS alternative

* Add pseudo-symbols: :external, :never -- sugar for [^\d\D]

* Problem with indirect nulling.  Modifying Semantics::Null to add explicit
  empty production changes semantics.  "Topmost nulled" rule seems not to be
  followed in this case.  This is a "later to do" because the same issue exists
  in the NAIF.

Work on semantics documentation: t+
Work on semantics documentation: t+

Ensure default for nulled start symbol is that when current where start
statement occurs in DSL source.
