EXAMPLES
     Print lines longer than 72 characters:

          length > 72

     Print first two fields in opposite order:

          { print $2, $1 }

     Add up first column, print sum and average:

               { s += $1 }
          END  { print "sum is", s, " average is", s/NR }

     Print fields in reverse order:

          { for (i = NF; i > 0; --i) print $i }

     Print all lines between start/stop pairs:

          /start/, /stop/

     Print all lines whose first field is different from previous
     one:

          $1 != prev { print; prev = $1 }