choose one of these missing value formalization options

[turbomam]

• any_of.range including a missing values enumeration (only for required slots that aren't intentionally open ended strings) (@cmungall's recommendation)
  • this may make it impossible to create linkml-sqldb dumps or RDF datasets
• structured values: numeric portion, unit, missing value code
• companion missing value code slots

would any of these require rules?

in which cases would these be applied to any slot other than non-open-ended, required?