Appending to a list variable X should append the succeeding value(s) to X
regardless of whether X is currently empty or not.

Before:

  acid: l={}
  acid: append l, 1
  {0x00000001}
  acid: l
  acid: 

With this patch:

  acid: l={}
  acid: append l, 1
  {0x00000001}
  acid: l
  {0x00000001}
  acid: