#13 Operations on sequences

[cchris]

1/ There should be a way to dynamically specify the
indexes of a sequence. Thus, if t is a sequence of
integers and s any sequence, we'd have:

s[[t]]=s[t[1]][t[2]]...[t[length(t)]]

2/ Also, a composition operator can be very handy to
translate indexes. If t is a sequence of integers and s
any sequence, we thus could have:

s配={s{t{1]],s[t[2]],...,s[t[length(t)]]}