Fix double stripping of HTML entities.

In SRM 596 div1 500, this happens in the generated problem statement:

For any two distinct elements A, B in S, A&B &gt; 0.

Instead of:

For any two distinct elements A, B in S, A&B > 0.

I tracked this issue to TopCoder's toXML(). So I had to add a function to detect this double stripping and fix it.

Although with remembrance of the annotation issue ,  I am starting to wonder if we are supposed to use toXML() for HTML at all. I don't know.