def get_elements(self, element_type, filter={}):
        if element_type not in self.elements:
            msg = "Unable to search for element %s in rspec, expath expression not found." % \
                   element_type
            raise InvalidRSpecElement(element_type, extra=msg)
        rspec_element = self.get_rspec_element(element_type)
        xpath = rspec_element.path + XpathFilter.xpath(filter)
        return self.xml.xpath(xpath)
search for a registered element
dataset/ETHPy150Open fp7-ofelia/ocf/ofam/src/src/foam/sfa/rspecs/rspec.py/RSpec.get_elements