code: private Result searchSubType(PType sub, PType sup, boolean invignore)
  	{
  		TypePair pair = new TypePair(sub, sup);
  		int i = done.indexOf(pair);
  
  		if (i >= 0)
  		{
  			return done.get(i).result; // May be "Maybe".
  		} else
  		{
  			done.add(pair);
  		}
  
  		// The pair.result is "Maybe" until this call returns.
  		pair.result = subtest(sub, sup, invignore);
  
  		return pair.result;
  	}
docs: Search the {@link #done} vector for an existing subtype comparison of two types before either returning the
  previous result, or making a new comparison and adding that result to the vector.
  
  @param sub
  @param sup
  @param invignore
  @return Yes or No, if sub is a subtype of sup.