code: void write(CallInfo callInfo, SimpleAssertion assertion) {
      Element rootNode = root(callInfo);
      if (assertion.getSource() != null) {
        write(rootNode, assertion.getSource());
      }
      Element saNode = createNode(rootNode, SIMPLE_ASSERTION_TAG);
      if (! assertion.passed()) {
        Element errorsNode = createNode(saNode, ERRORS_TAG);
        createNode(errorsNode, EXPECTED_TAG)
          .setAttribute(VALUE_ATTR, assertion.getExpectedResult().toString());
        createNode(errorsNode, ACTUAL_TAG)
          .setAttribute(VALUE_ATTR, assertion.getActualResult().toString());
      }
      flush(rootNode);
    }
docs: Log simple assertion.
  @param callInfo Call info.
  @param assertion Delta assertion.