Select Code to Download

# Command-line
(my $x = shift) or die $syntax;
...
# Validity checking
($x =~ /^-?\d+$/) or die "$iam:  value $x not an integer\n";
($y =~ /^-?\d+$/) or die "$iam:  value $y not an integer\n";

my $start = shift @ARGV;
my $end = shift @ARGV;