title: Join values from two specific rows as a new row and values separated by semicolon using dplyr
body: <p>Similar to <code>unite()</code> do to columns, is there a way to combine rows across columns for specific rows separating values with semicolon?</p>
  <p>In the example below, <code>IC_1</code> and <code>IC_2</code> were combined as a new row, values between brackets and separeted by <code>;</code></p>
  <pre><code>structure(list(treatment = c(&quot;product&quot;, &quot;product&quot;, &quot;product&quot;, 
  &quot;product&quot;, &quot;control&quot;, &quot;control&quot;, &quot;control&quot;, &quot;control&quot;), variable = c(&quot;A&quot;, 
  &quot;B&quot;, &quot;IC_1&quot;, &quot;IC_2&quot;, &quot;A&quot;, &quot;B&quot;, &quot;IC_1&quot;, &quot;IC_2&quot;), X1 = 1:8, X2 = 8:15, 
      X3 = 16:23), class = c(&quot;tbl_df&quot;, &quot;tbl&quot;, &quot;data.frame&quot;), row.names = c(NA, 
  -8L))
  
   treatment variable    X1    X2    X3
    &lt;chr&gt;     &lt;chr&gt;    &lt;int&gt; &lt;int&gt; &lt;int&gt;
  1 product   A            1     8    16
  2 product   B            2     9    17
  3 product   IC_1         3    10    18
  4 product   IC_2         4    11    19
  5 control   A            5    12    20
  6 control   B            6    13    21
  7 control   IC_1         7    14    22
  8 control   IC_2         8    15    23 
  </code></pre>
  <p>Desired output:</p>
  <pre><code>treatment variable X1    X2    X3   
    &lt;chr&gt;     &lt;chr&gt;    &lt;chr&gt; &lt;chr&gt; &lt;chr&gt;
  1 product   A        1     8     16   
  2 product   B        2     9     17   
  3 product   IC     [3;4][10;11][18:19]
  4 control   A        5     12    20   
  5 control   B        6     13    21   
  6 control   IC     [7;8][14;15][22;23]
  </code></pre>