code: public function register($alias, $abstract)
      {
          if (!is_string($alias) || !is_string($abstract)) {
              throw new \InvalidArgumentException(
                  sprintf("Parameter 1 and 2 of %s must be a string.", __METHOD__)
              );
          }
  
          if (!isset($this->aliases[$alias])) {
              $this->aliases[$alias] = $this->make($abstract);
          }
  
          return $this;
      }
docs: Register a service alias.
  
  @param string $alias The alias name.
  @param string $abstract The class name.