protected function assert($label, $clause)
{
$pc = new PrologCompiler($this);
$prog = $pc->compileSimpleClause($clause . ".");
if ($prog != null) {
$this->p->addClause($label, $prog);
$this->programCounter++;
$v = new Variable("", $label);
$v->tag = self::ASSERT;
$this->trail->addEntry($v);
} else {
$this->backtrack();
}
}
