roman.rb

@ids = [[1,'I'],[5,'V'],[10,'X'],[50,'L'],[100,'C'],[500,'D'],[1000,'M']]

def to_roman(num)
  str = ''
  @ids.reverse.each { |id|
    until num / id[0] == 0
      str += id[1]
      num -= id[0]
    end
    if num == id[0]-1 && check[0] != 1
      str += "I" + id[1]
      num -= id[0]-1
    end
  }
  return str
end

def err_msg(num,expected)
  return "Error! #{num} => #{to_roman(num).inspect} Expected #{num} => #{expected.inspect}"
end

def do_test(num,expected)
  return to_roman(num) == expected ? true : err_msg(num,expected)
end

tests = [[2,"II"],[5,"V"],[9,"IX"],[10,"X"],[11,"XI"],[15,"XV"],[19,"XIX"]]
tests += [[50,"L"],[60,"LX"],[65,"LXV"],[69,"LXIX"],[100,"C"],[600,"DC"]]
tests << [1669,"MDCLXIX"]

puts "Testing Identities"
@ids.each { |id| puts do_test(id[0],id[1]) }
puts "Testing Non Identities"
tests.each { |test| puts do_test(test[0],test[1]) }

By changing the ids array you can create now identities (which will automatically be tested!)