/hl7.org/fhir/R4/conformance-rules.html#ismodifier">Is Modifier</a></td><td><span style="color:DarkGray">false</span></td></tr><tr><td style="white-space: nowrap">Invariants</td><td><span style="color:DarkGray"><b title="Formal Invariant Identifier">ele-1</b>: All FHIR elements must have a @value or children (hasValue() or (children().count() &gt; id.count()))</span><br/><b title="Formal Invariant Identifier">ext-1</b>: Must have either extensions or value[x], not both (<code>extension.exists() != value.exi