(VAR X Y Z X1 X2)
(RULES 
active(dbl(0)) -> mark(0)
active(dbl(s(X))) -> mark(s(s(dbl(X))))
active(dbls(nil)) -> mark(nil)
active(dbls(cons(X,Y))) -> mark(cons(dbl(X),dbls(Y)))
active(sel(0,cons(X,Y))) -> mark(X)
active(sel(s(X),cons(Y,Z))) -> mark(sel(X,Z))
active(indx(nil,X)) -> mark(nil)
active(indx(cons(X,Y),Z)) -> mark(cons(sel(X,Z),indx(Y,Z)))
active(from(X)) -> mark(cons(X,from(s(X))))
mark(dbl(X)) -> active(dbl(mark(X)))
mark(0) -> active(0)
mark(s(X)) -> active(s(X))
mark(dbls(X)) -> active(dbls(mark(X)))
mark(nil) -> active(nil)
mark(cons(X1,X2)) -> active(cons(X1,X2))
mark(sel(X1,X2)) -> active(sel(mark(X1),mark(X2)))
mark(indx(X1,X2)) -> active(indx(mark(X1),X2))
mark(from(X)) -> active(from(X))
dbl(mark(X)) -> dbl(X)
dbl(active(X)) -> dbl(X)
s(mark(X)) -> s(X)
s(active(X)) -> s(X)
dbls(mark(X)) -> dbls(X)
dbls(active(X)) -> dbls(X)
cons(mark(X1),X2) -> cons(X1,X2)
cons(X1,mark(X2)) -> cons(X1,X2)
cons(active(X1),X2) -> cons(X1,X2)
cons(X1,active(X2)) -> cons(X1,X2)
sel(mark(X1),X2) -> sel(X1,X2)
sel(X1,mark(X2)) -> sel(X1,X2)
sel(active(X1),X2) -> sel(X1,X2)
sel(X1,active(X2)) -> sel(X1,X2)
indx(mark(X1),X2) -> indx(X1,X2)
indx(X1,mark(X2)) -> indx(X1,X2)
indx(active(X1),X2) -> indx(X1,X2)
indx(X1,active(X2)) -> indx(X1,X2)
from(mark(X)) -> from(X)
from(active(X)) -> from(X)
)