Currently, this extension is a a flag that contains a fixed SNOMED code.   If it is present, the Patient is a deceased fetus.   This is much more complicated than the deceased flag which contains Boolean or a dateTime.

MichaelRiley noted the excessive complexity of the extension, and the fact that it can't be set to false.   

Suggest:
  1) making it a simple boolean valued extension on deceased
  2) restricting it to being true iff deceased is true
  3) implied value of false if absent, but it can be set to false.