> 
> 
> 
> #Q1. THE FIRST ATTENDANCE QUESTION WAS:  What is a derangement? Learn by wikipedia
> 
> 
> #A1. MY ANSWER TO THE FIRST ATTENDANCE QUESTION IS:   A derangement is a permutation of the elements of a set such that no element appears in its original position. In other words, a derangement is permutation that has no fixed points.
> 
> 
> 
> #Q2. THE  SECOND ATTENDANCE QUESTION WAS: Give two examples of Wtos and StoW. 
> 
> 
> #A2. MY ANSWER TO THE  SECOND ATTENDANCE QUESTION IS:
> WtoS:=proc(w) local n,i,S:
> 
> n:=nops(w):
> 
> S:={}:
> 
> for i from 1 to n do
>  if w[i]=1 then
>   S:=S union {i}:
>  fi:
> od:
> 
> S:
> 
> end:
> 
> 

;
> StoW:=proc(S,n) local i,w:
> 
> if S  minus {seq(i,i=1..n)}<>{} then
>  print(S, `is not a subset of`,  {seq(i,i=1..n)} ):
>  RETURN(FAIL):
> fi:
> 
> w:=[]:
> 
> for i from 1 to n do
>  
> if member(i,S) then
>   w:=[op(w),1]:
> else
>   w:=[op(w),0]:
> fi:
> 
> od:
> 
> w:
> 
> end:
> 
;
> #My example:
> WtoS([1,0,1,1,0,0,1]);
{1, 3, 4, 7}

;
> WtoS([0,0,0,0,0]);
{}

;
> StoW({},5);
[0, 0, 0, 0, 0]

;
> StoW({1,3,4,7},6);
{1, 3, 4, 7}, `is not a subset of`, {1, 2, 3, 4, 5, 6}
FAIL

;
> StoW({1,3,4,7},7);
[1, 0, 1, 1, 0, 0, 1]

;
> 
;