Scoping of local variables in Ruby

gistfile1.txt

colinux> ruby local.rb
foo(true):
  a is not defined.
  a is defined.
  a = 1

foo(false):
  a is not defined.
  a is defined.
  a = nil

local.rb

#!/usr/bin/ruby1.9.1 -Ku
# -*- mode: ruby; coding: utf-8-unix -*-

def foo(t)
   printf "foo(%s):\n", t.inspect

   if defined?(a) then
      printf " a is defined.\n"
   else
      printf " a is not defined.\n"
   end

   if t then
      a = 1
   end

   if defined?(a) then
      printf " a is defined.\n"
   else
      printf " a is not defined.\n"
   end

   printf " a = %s\n", a.inspect
   printf "\n"
end

foo(true)
foo(false)