title: Dplyr mutate new column at a specified location
body: <p>An example:</p>
  
  <pre><code>a = c(10,20,30)
  b = c(1,2,3)
  c = c(4,5,6)
  d = c(7,8,9)
  df=data.frame(a,b,c,d)
  
  library(dplyr)
  
  df_1 = df %&gt;% mutate(a1=sum(a+1))
  </code></pre>
  
  <p>How do I add "a1" after "a" (or any other defined position) and NOT at the end?</p>
  
  <p>Thank you.</p>