code: public IntStreamEx mapFirst(IntUnaryOperator mapper) {
          return delegate(new PairSpliterator.PSOfInt((a, b) -> b, mapper, spliterator(),
                  PairSpliterator.MODE_MAP_FIRST));
      }
docs: Returns a stream where the first element is the replaced with the result
  of applying the given function while the other elements are left intact.
  
  <p>
  This is a <a href="package-summary.html#StreamOps">quasi-intermediate
  operation</a>.
  
  @param mapper a
  <a href="package-summary.html#NonInterference">non-interfering
  </a>, <a href="package-summary.html#Statelessness">stateless</a>
  function to apply to the first element
  @return the new stream
  @since 0.4.1