[plt-scheme] HTDP Excercise 14.1.5

Thanks I got it now.I have one question though , in the documentation of
append it is given that

(append<file:///usr/local/plt/doc/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._append%29%29>
 lst ...) → list?<file:///usr/local/plt/doc/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._list%7E3f%29%29>
  lst : list?<file:///usr/local/plt/doc/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._list%7E3f%29%29>
(append<file:///usr/local/plt/doc/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._append%29%29>
 lst ... v) → any/c<file:///usr/local/plt/doc/reference/Data-structure_Contracts.html#%28def._%28%28lib._scheme/private/contract-guts..ss%29._any/c%29%29>
  lst : list?<file:///usr/local/plt/doc/reference/pairs.html#%28def._%28%28quote._%7E23%7E25kernel%29._list%7E3f%29%29>
  v : any/c<file:///usr/local/plt/doc/reference/Data-structure_Contracts.html#%28def._%28%28lib._scheme/private/contract-guts..ss%29._any/c%29%29>

When given all list arguments, the result is a lists that contains all of
the elements of the given lists in order. The last argument is used directly
in the tail of the result.
The last argument need not be a list, in which case the result is an
“improper list.


Can someone please explain what does an improper list means?


Aditya
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