function find_first()
{
$dot = count($this->rhs);
$last = $this->first[$dot] = new set();
while ($dot) {
$dot--;
$symbol_after_the_dot = $this->rhs[$dot];
$first = $symbol_after_the_dot->first->all();
bug_if(empty($first) and !$symbol_after_the_dot->lambda);
$set = new set($first);
if ($symbol_after_the_dot->lambda) {
$set->union($last);
if ($this->epsilon == $dot + 1) {
$this->epsilon = $dot;
}
}
$last = $this->first[$dot] = $set;
}
}
