title: Chisel memory write mask
body: <p>I am trying to use a write mask for a Mem in chisel 3 like given below.</p>
  
  <pre><code>chipMem.write(data_idx, wdata, wmask)
  </code></pre>
  
  <p>I am generating wmask (the write mask) as follows,</p>
  
  <pre><code>val wmask = write_mask.toBools
  </code></pre>
  
  <p>since write_mask is a UInt and the write function requires a Bool Sequence for the write mask argument. However, this gives the following error.</p>
  
  <pre><code>Cannot prove that chisel3.core.UInt &lt;:&lt; chisel3.core.Vec[_].
  [error]              chipMem.write(data_idx, wdata, wmask)
  [error]                           ^
  [error] one error found
  </code></pre>
  
  <p>I really don't understand what the error message says. Is it a problem with the way I create wmask?</p>