如何在不安装整个pod的情况下安装pod的子规范？

我只想要一个库的一个子规格,例如,nimbus podspec如下:

pod search Nimbus

-> Nimbus (1.0.0)

An iOS framework whose growth is bounded by O(documentation).
- Homepage: http://docs.nimbuskit.info/index.html
- Source:   https://github.com/jverkoey/nimbus.git
- Versions: 1.0.0, 0.9.3, 0.9.2, 0.9.1, 0.9.0 [master repo]
- Sub specs:
    - Nimbus/Core (1.0.0)
    - Nimbus/Badge (1.0.0)
    - Nimbus/CSS (1.0.0)
    - Nimbus/AttributedLabel (1.0.0)
    - Nimbus/Interapp (1.0.0)
    - Nimbus/Launcher (1.0.0)
    - Nimbus/Models (1.0.0)
    - Nimbus/NetworkControllers (1.0.0)
    - Nimbus/NetworkImage (1.0.0)
    - Nimbus/Overview (1.0.0)
    - Nimbus/PagingScrollView (1.0.0)
    - Nimbus/Photos (1.0.0)
    - Nimbus/Operations (1.0.0)
    - Nimbus/Operations/JSON (1.0.0)
    - Nimbus/WebController (1.0.0)

我只是想安装Nimbus/core,应该podfile包含哪些内容？