Write a page program
so that page f
prints on stdout the first 20 lines of the file named f,
then reads and discards a line from stdin,
then prints on stdout the next 20 lines of the file,
then reads and discards another line from stdin,
etc.

Stop immediately after printing the last line of the file.
If there is any error in opening the file,
reading from the file,
reading from stdin,
or writing to stdout,
print a detailed description of the problem on stderr and stop.