foo.coffee

# Create a new class A with instance method a
A = Em.Object.extend
  a: -> 1

# Create a new class B with instance method b
B = Em.Object.extend
  b: -> 2

# Create a new class C with instance methods b & c
C = B.extend
 c: -> 3

# Add a new method to C called d
C.reopen
 d: -> 4
 
# Add a class method to c, that you can call with c.e()
C.reopenClass
 e: -> 5