title: how to understand these vim scripts
body: <p>I have two question about understand those vim script. please give some help,</p>
  
  <p>Question 1:
  I download a.vim plugin, and i try to read this plugin, how to understand the below variable definition?  the first line I can understand, but the second line, I don't know exactly "g:alternateExtensions_{'aspx.cs'}" means.</p>
  
  <pre><code>" E.g. let g:alternateExtensions_CPP = "inc,h,H,HPP,hpp" 
  "      let g:alternateExtensions_{'aspx.cs'} = "aspx"
  </code></pre>
  
  <p>Question 2:
     how to understand "SID" before the function name, using like below function definition and function call.</p>
  
  <pre><code>function! &lt;SID&gt;AddAlternateExtensionMapping(extension, alternates)
  //omit define body
  
  call &lt;SID&gt;AddAlternateExtensionMapping('h',"c,cpp,cxx,cc,CC")
  call &lt;SID&gt;AddAlternateExtensionMapping('H',"C,CPP,CXX,CC")
  </code></pre>
  
  <p>thanks for you kindly help.</p>