is_empty
nil
true
is_empty
cons
x
l
false
hd
cons
x
l
x
tl
cons
x
l
l
append
l1
l2
ifappend
l1
l2
is_empty
l1
ifappend
l1
l2
true
l2
ifappend
l1
l2
false
cons
hd
l1
append
tl
l1
l2
is_empty
1
nil
0
true
0
cons
2
false
0
hd
1
tl
1
append
2
ifappend
3
INNERMOST
./TRS/Cime/append-hard.trs