Fix issue #25 (simplification of unary expressions)

Also add a few related tests

src/language/ast/ASTBuilder.cpp

@@ -74,7 +74,7 @@ struct ASTBuilder::simplify_unary : TAO_PEGTL_NAMESPACE::parse_tree::apply<ASTBu
           transform(n, st...);
         }
       }
-    } else if (n->children.size() == 2) {
+    } else if ((n->children.size() == 2) and (n->is_type<language::unary_expression>())) {
       if (n->children[0]->is_type<language::unary_plus>()) {
         n = std::move(n->children[1]);
         transform(n, st...);