Sorry I am trying really hard to get my head around this but I am failing.

I get that j=1...k is the part which determines the size of each list, in this case going up to 5

however I really dont get the part    (`if`((j+k)::even,s,t),j=1..k)], is it saying if j+k is even then the element is s, if not it is t

i.e [[ 1+1=even therefore s], [2+1=odd therfore t, 1+1=even therefore s], [ 3+1=even hence s] etc.....

sorry I am just very confused.

Sorry I am a little hopeless when it comes to maple.

converting to a set makes sense but I am not sure how it would work with teh example I have

However in correspondance to the first reply from jpmay i have tried to come up with a procedure however I know it is not correct and was hoping someone would be able to help.

winnow1:= proc(L)
local i;
for i in L do
if member( L[i], L, 'j') then # 3 argument form of member
subsop(i1=NULL, i2=NULL, ..., ik=NULL, L) # removes duplicate entries
end if
end do;
return L
end proc:

Thanks again.