public function step($result, $input)
{
$pval = $this->pval;
$val = call_user_func($this->f, $input);
$this->pval = $val;
$none = Transducers::none();
if ($pval === $none || $pval === $val) {
$this->a[] = $input;
return $result;
}
$ret = $this->xf->step($result, $this->a);
$this->a = [];
if (!Transducers::isReduced($ret)) {
$this->a[] = $input;
}
return $ret;
}
