to solve the following bug:

f
  # x = undef
  #! y = x
  | True
    = 2
  = y

After checking this will be like

f
  # x = undef
  | True
    = 2
  = x

To really solve this bug we have to invent something new, because aliases
like (#! y = x) are _not_ allowed. We could either introduce a dummy identity function:

f
  # x = undef
  #! y = dummy_id x
  | True
    = 2
  = y

or we could introduce constructs like

f
  # x = undef
  #! y
  | True
    = 2
  = y