title: na.locf remove leading NAs, keep others
body: <p>I have a question regarding the <code>na.locf</code> function in the <code>zoo</code> package. Within the data frame below I want to remove the leading NAs (for years 1987, 1988) but keep those with a valid value for the previous year (1993). </p>
  
  <pre><code>Year     X
  1987     NA
  1988     NA
  1989     2
  1990     5
  1991     9
  1992     16
  1993     NA
  1994     27
  1995     36
  </code></pre>
  
  <p>Does anyone have a solution for this problem?</p>