BMI with links to Height and Weight (id = "bmi-using-related")

<?xml version="1.0" encoding="UTF-8"?>

<Observation xmlns="http://hl7.org/fhir">
  <id value="bmi-using-related"/> 
  <!--   ======== need to fix vitals to removed fixed value 'has-member' ==========
  <meta>
    <profile value="http://hl7.org/fhir/StructureDefinition/vitalsigns"/>
  </meta>
      -->
  <status value="final"/> 
  <category> 
    <coding> 
      <system value="http://terminology.hl7.org/CodeSystem/observation-category"/> 
      <code value="vital-signs"/> 
      <display value="Vital Signs"/> 
    </coding> 
    <text value="Vital Signs"/> 
  </category> 
  <category> 
    <coding> 
      <system value="http://loinc.org"/> 
      <code value="39156-5"/> 
      <display value="Body mass index (BMI) [Ratio]"/> 
    </coding> 
  </category> 
  <code> 
    <coding> 
      <system value="http://loinc.org"/> 
      <code value="39156-5"/> 
      <display value="Body mass index (BMI) [Ratio]"/> 
    </coding> 
    <text value="BMI"/> 
  </code> 
  <subject> 
    <reference value="Patient/example"/> 
  </subject> 
  <effectiveDateTime value="1999-07-02"/> 
  <valueQuantity> 
    <value value="16.2"/> 
    <unit value="kg/m2"/> 
    <system value="http://unitsofmeasure.org"/> 
    <code value="kg/m2"/> 
  </valueQuantity> 
  <derivedFrom> 
    <reference value="Observation/bodyheight"/> 
    <display value="Body Height"/> 
  </derivedFrom> 
  <derivedFrom> 
    <reference value="Observation/example"/> 
    <display value="Body Weight"/> 
  </derivedFrom> 
</Observation>