code: def parse_time_derivative(self, node):
          
  
          if 'variable' in node.lattrib:
              variable = node.lattrib['variable']
          else:
              self.raise_error('<TimeDerivative> must specify a variable.')
  
          if 'value' in node.lattrib:
              value = node.lattrib['value']
          else:
              self.raise_error("Time derivative for '{0}' must specify an expression.",
                               variable)
  
          self.current_regime.add_time_derivative(TimeDerivative(variable, value))
docs: Parses <TimeDerivative>
  
          @param node: Node containing the <TimeDerivative> element
          @type node: xml.etree.Element
  
          @raise ParseError: Raised when the time derivative does not hava a variable
          name of a value.