public function toBe()
    {
        if ($this->reference()) {
            $this->_substitutes = func_get_args();
        } else {
            $this->_actsAsAReplacement = true;
            if (func_num_args() !== 1) {
                throw new Exception("Only one hard method substitution is allowed through `toBe()`.");
            }
            $this->_closures = func_get_args();
        }
    }