// We have this enum.
enum Formula {
  case proposition(String)
  indirect case negation(Formula)
  indirect case operation(op: String, lhs: Formula, rhs: Formula)

  var nnf: Formula { /* ... */  }
}

// And now ...

switch formula.nnf {
 case .proposition(_):
   return formula.nnf
 case .negation(_):
   return formula.nnf
 case .operation(_, _, _):
   return formula.nnf
}