diff --git a/resources/schema/inventory.xsd b/resources/schema/inventory.xsd
index e47499a..7a408cc 100644
--- a/resources/schema/inventory.xsd
+++ b/resources/schema/inventory.xsd
@@ -262,7 +262,7 @@
   <!-- Mixed-content container element used in examples where roles can include multiple word components (e.g., semantic roles).   -->
   <xsd:complexType name="exampleTreeType" mixed="true">
     <xsd:sequence>
-      <xsd:element name="comp" type="compType" maxOccurs="unbounded"/> <!-- the tree contains one or more components -->
+      <xsd:element name="comp" type="compType" minOccurs="0" maxOccurs="unbounded"/> <!-- the tree contains one or more components -->
     </xsd:sequence>
     <xsd:attribute name="role" type="semanticRoleType"/>
   </xsd:complexType>
@@ -276,6 +276,7 @@
       </xsd:choice>
     </xsd:sequence>
     <xsd:attribute name="corpus_id" type="xsd:int"/>
+    <xsd:attribute name="source" type="xsd:string"/>
     <xsd:attribute name="example_id" type="xsd:int"/>
     <xsd:attribute name="exampleId" type="xsd:string"/> <!-- the example's ID within the corpus itself --> 
     <xsd:attribute name="modified" type="xsd:boolean"/> <!-- is the example text different than in the corpus? -->
@@ -433,7 +434,7 @@
   <xsd:complexType name="semanticRoleContainerType">
     <xsd:sequence>
       <xsd:element name="semanticRole" type="semanticRoleType"/>
-      <xsd:element name="syntacticStructureList" type="syntacticStructureListType"/>
+      <xsd:element name="syntacticStructureList" type="syntacticStructureListType" minOccurs="0" maxOccurs="1"/>
     </xsd:sequence>
     <xsd:attribute name="cluster" type="xsd:int"/>  <!-- hmm, number identifying a group of clustered roles? -->
   </xsd:complexType>
@@ -466,6 +467,14 @@
     <xsd:sequence>
       <xsd:element name="valencyPattern" type="valencyPatternType" minOccurs="0" maxOccurs="unbounded"/>
     </xsd:sequence>
+    <xsd:attribute name="system"> <!-- the system within which the structure is defined  -->
+      <xsd:simpleType>
+        <xsd:restriction base="xsd:string">
+          <xsd:enumeration value="JOS"/>
+          <xsd:enumeration value="UD"/>
+        </xsd:restriction>
+      </xsd:simpleType>
+    </xsd:attribute>
   </xsd:complexType>
 
 </xsd:schema>