Getting private member functions working in CoffeeScript by scoping differently breaks inheritance

gistfile1.coffee

# This is an example in a post about private members in CoffeeScript.
# Read more: http://evanhahn.com/?p=1126

class Sorcerer
  that = null
  constructor: (@spell) ->
    that = this
  conjureSpell = ->   # private
    that.spell.conjure()
  useSpell: ->
    conjureSpell()
    @spell.use()

class Child extends Sorcerer
  constructor: (@spell) -> # Overriding the constructor, only works if you say "that = this" in here

c = new Child
  conjure: -> console.log "Brewing potion..."
  use: -> console.log "Now I have superpowers!"

c.useSpell()  # Cannot read property 'spell' of null

# This code by Evan Hahn (evanhahn.com) is licensed under the Unlicense.
# http://unlicense.org/