title: How to set values to np.nan with multiple conditions for series?
body: <p>Let's say I have a dataframe:</p>
  <pre><code>    A   B   C   D   E   F
  0   x   R   i   R   nan h
  1   z   g   j   x a   nan
  2   z   h   nan y nan nan
  3   x   g   nan nan nan nan
  4   x   x   h   x   s   f
  </code></pre>
  <p>I want to replace all the cells where:</p>
  <ol>
  <li>the value in row 0 is R (<code>df.loc[0] == 'R'</code>)</li>
  <li>the cell is not 'x' (<code>!= 'x'</code>)</li>
  <li>only rows 2 and below (2:)</li>
  </ol>
  <p>with <code>np.nan</code>.</p>
  <p>Essentially I want to do:</p>
  <pre><code>df.loc[2:,df.loc[0]=='R']!='x' = np.nan
  </code></pre>
  <p>I get the error:</p>
  <pre><code>SyntaxError: can't assign to comparison
  </code></pre>
  <p>I just don't know how the syntax is supposed to be.</p>
  <p>I've tried</p>
  <pre><code>df[df.loc[2:,df.loc[0]=='R']!='x']
  </code></pre>
  <p>but this doesn't list the values I want.</p>