title: find a column value in other columns in a pandas dataframe with apply?
body: <p>I have a <code>df</code> that has the following form:</p>
  
  <pre><code>        id  Tg n0  n1  n2
  fddef72025   W  A   B   C
  fddef72025   A  A   B   C
  fddef72025   Y  A   B   C
  fddef72025   B  A   B   C
  </code></pre>
  
  <p>I need to create a new column and store a 1 if <code>Tg</code> is in either <code>df.n0</code>, <code>df.n1</code> or <code>df.n2</code>, so for example for the <code>df</code> above:</p>
  
  <pre><code>        id  Tg n0  n1  n2  exists
  fddef72025   W  A   B   C       0
  fddef72025   A  A   B   C       1
  fddef72025   Y  A   B   C       0
  fddef72025   B  A   B   C       1
  </code></pre>
  
  <p>My solution so far involves "iterrows", but I'm sure there is a way to <code>df.apply</code> this?</p>