Dummy variable:
@example
a = 3
  @result{} a = 3
@end example

The following test will fail:
@example
@c doctest: +XFAIL_IF(6 + 0*now () >= 0)
a = 5
  @result{} b = 7
@end example

value is changed (b/c xfail, not skip)
@example
a
  @result{} a = 5
@end example
