Any one able to tell me why this won't work? 

cc:=proc(M,shift)
local x,string;
string:=expl(M);
for x in string do
cl(x,shift);
end do; end proc; 

expl:=proc(S)
Explode(S);
end proc;

cc(ok,1) returns 'l'

expl works, how come the procedure using expl doesn't explode the message and do the procedure on each letter?

As a college project we have to implement the caesar cipher on maple but as beginners we haven't got a clue. 

Any advice even on how to begin please?