Update documentation and fix Relax NG schema

docs/source/usersguide/input.rst

@@ -906,6 +906,10 @@ Each ``<surface>`` element can have the following attributes or sub-elements:
 
     *Default*: "transmission"
 
+  :periodic_surface_id:
+     If a periodic boundary condition is applied, this attribute identifies the
+     ``id`` of the corresponding periodic sufrace.
+
 The following quadratic surfaces can be modeled:
 
   :x-plane:

src/relaxng/geometry.rnc

@@ -23,7 +23,7 @@ element geometry {
     (element coeffs { list { xsd:double+ } } | attribute coeffs { list { xsd:double+ } }) &
     (element boundary { ( "transmit" | "reflective" | "vacuum" | "periodic" ) } |
      attribute boundary { ( "transmit" | "reflective" | "vacuum" | "periodic" ) })? &
-    (element opposite { xsd:int } | attribute opposite { xsd:int })?
+    (element periodic_surface_id { xsd:int } | attribute periodic_surface_id { xsd:int })?
   }*
 
   & element lattice {

src/relaxng/geometry.rng

@@ -188,10 +188,10 @@
           </optional>
           <optional>
             <choice>
-              <element name="opposite">
+              <element name="periodic_surface_id">
                 <data type="int"/>
               </element>
-              <attribute name="opposite">
+              <attribute name="periodic_surface_id">
                 <data type="int"/>
               </attribute>
             </choice>