#! /usr/bin/env ruby
#
# Generate prefixes.
#
#   % prefixes abcdefg
#   a
#   ab
#   abc
#   abcd
#   abcde
#   abcdef
#   abcdefg

exit(1) if ARGV.empty?
str = ARGV[0]
str.length.times {|i|
  puts str[0,i+1]
}