docs: def toxml(self):
          """
          Exports this object into a LEMS XML object
          """
  
          return '<DerivedVariable name="{0}"'.format(self.name) +\
            (' dimension="{0}"'.format(self.dimension) if self.dimension else '') +\
            (' exposure="{0}"'.format(self.exposure) if self.exposure else '') +\
            (' select="{0}"'.format(self.select) if self.select else '') +\
            (' value="{0}"'.format(self.value) if self.value else '') +\
            (' reduce="{0}"'.format(self.reduce) if self.reduce else '') +\
            (' required="{0}"'.format(self.required) if self.required else '') +\
            '/>'