Primitive Domains

• What happens if we try to evaluate

  substr ((A concat (B concat C)), one , four)
  
  We need to use error => add an error element to String and
  extend all other operations appropriately.
  

• The one element domain

  Domain Unit,
  
  Operations
  ( ) : Unit
  
  This domain may be used whenever an operation needs a dummy
  arguments e.g.let f: Unit ? Nat be f(x) = one; thus f() = one.

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