ahead: filter the first lines ('head') of the input and print them

Parameters:
-n [count]: only the first 'count' lines will be forwarded to the output
-e [expression]: only the first lines will be printed that give !0 result
  with this expression (integer result != 0). The 'line' runtime variable
  contains the line.
-until: default mode, lines are forwarded until the expression is >0
-while: lines are forwarded while the expression is >0

Examples:
afind *.txt | acat | ahead -n 10
  -> pint the first 10 lines of each text file

... | ahead -e "strlen(line) > 10"
  -> print the first lines until the line length reaches 10
    (the expression quoted, otherwise the command line interpreter would
    process the '>' and forward the results into the file '10')

... | ahead -e "strpos(line, 'apple') != 0"
  -> print the first lines that contain the "apple" string

... | ahead -e "line []= 'apple'"
  -> the same as the previous one