Return Position relative to current position in last(), first().

It was returning Position based off absolute position, which
was no good for operators like orth[first(M)] working in changing
sentence contexts.

libwccl/ops/functions/position/firsttoken.cpp

@@ -21,7 +21,9 @@ FirstToken::BaseRetValPtr FirstToken::apply_internal(
 	if(match->empty()) {
 		return detail::DefaultFunction<Position>()->apply(context);
 	}
-	return boost::make_shared<Position>(match->first_token(s));
+	int abs_pos = match->first_token(s);
+	int rel_pos = abs_pos - context.sentence_context().get_position();
+	return boost::make_shared<Position>(rel_pos);
 }
 
 std::string FirstToken::to_string(const Corpus2::Tagset &tagset) const

libwccl/ops/functions/position/lasttoken.cpp

@@ -14,13 +14,15 @@ LastToken::BaseRetValPtr LastToken::apply_internal(
 	if (!s) {
 		throw InvalidArgument(
 			"context",
-			"Supplied context does not have valid Corpus2::AnnotatedSentence.");
+			"Supplied context does not have a valid Corpus2::AnnotatedSentence.");
 	}
 	const Function<Match>::RetValPtr match = match_expr_->apply(context);
 	if(match->empty()) {
 		return detail::DefaultFunction<Position>()->apply(context);
 	}
-	return boost::make_shared<Position>(match->last_token(s));
+	int abs_pos = match->last_token(s);
+	int rel_pos = abs_pos - context.sentence_context().get_position();
+	return boost::make_shared<Position>(rel_pos);
 }
 
 std::string LastToken::to_string(const Corpus2::Tagset &tagset) const