public function offsetGet($offset)
    {
        $offset = $this->calculateOffset($offset);
        return isset($this->_children[$offset]) ? $this->_children[$offset] : null;
    }
public function offsetGet($offset)
    {
        $offset = $this->calculateOffset($offset);
        return isset($this->_children[$offset]) ? $this->_children[$offset] : null;
    }
Returns a child token at the specified offset.
This method is required by the SPL [[\ArrayAccess]] interface.
It is implicitly called when you use something like `$child = $token[$offset];`.
@param int $offset child token offset.
@return SqlToken|null the child token at the specified offset, `null` if there's no token.