title: What is meaning of "import weak" in Protocol Buffers Version 2?
body: <p><a href="https://developers.google.com/protocol-buffers/docs/reference/proto2-spec#import_statement" rel="nofollow">Import Statement</a> in Protocol Buffers Version 2 Language Specification has the following text:</p>
  
  <pre><code>import = "import" [ "weak" | “public”] strLit ";" 
  </code></pre>
  
  <p>But <a href="https://developers.google.com/protocol-buffers/docs/proto#importing-definitions" rel="nofollow">Importing Definitions</a> in Protocol Buffers Version 2 Language Guide only talks about the use of <code>public</code>. What is the meaning of <code>import weak</code>? I have googled for an answer. But I fails to find an explanation.</p>