choice inside choice

```
If a mandatory choice is inside another choice, Schematron may report a false
positive because the first bullet in Sec. 7.9.4 isn't properly implemented in
gen-schematron.xsl. It happens e.g. for the following module:

module mandchoice {
namespace "http://example.com/ns/mandchoice";
prefix mc;

container top {
choice foobar {
case foo {
leaf baz {
type uint8;
}
choice one-two {
mandatory true;
leaf foo1 {
type uint8;
}
leaf foo2 {
type uint8;
}
}
}
leaf bar {
type uint8;
}
}
}
}

Schematron requires either "foo1" or "foo2" to be present even if leaf "baz" is
not present.
```

Original issue reported on code.google.com by `lada.lho...@gmail.com` on 3 Apr 2014 at 8:32