(0) Obligation:

Runtime Complexity Relative TRS:
The TRS R consists of the following rules:

#less(@x, @y) → #cklt(#compare(@x, @y))
append(@l1, @l2) → append#1(@l1, @l2)
append#1(::(@x, @xs), @l2) → ::(@x, append(@xs, @l2))
append#1(nil, @l2) → @l2
flatten(@t) → flatten#1(@t)
flatten#1(leaf) → nil
flatten#1(node(@l, @t1, @t2)) → append(@l, append(flatten(@t1), flatten(@t2)))
flattensort(@t) → insertionsort(flatten(@t))
insert(@x, @l) → insert#1(@l, @x)
insert#1(::(@y, @ys), @x) → insert#2(#less(@y, @x), @x, @y, @ys)
insert#1(nil, @x) → ::(@x, nil)
insert#2(#false, @x, @y, @ys) → ::(@x, ::(@y, @ys))
insert#2(#true, @x, @y, @ys) → ::(@y, insert(@x, @ys))
insertionsort(@l) → insertionsort#1(@l)
insertionsort#1(::(@x, @xs)) → insert(@x, insertionsort(@xs))
insertionsort#1(nil) → nil

The (relative) TRS S consists of the following rules:

#cklt(#EQ) → #false
#cklt(#GT) → #false
#cklt(#LT) → #true
#compare(#0, #0) → #EQ
#compare(#0, #neg(@y)) → #GT
#compare(#0, #pos(@y)) → #LT
#compare(#0, #s(@y)) → #LT
#compare(#neg(@x), #0) → #LT
#compare(#neg(@x), #neg(@y)) → #compare(@y, @x)
#compare(#neg(@x), #pos(@y)) → #LT
#compare(#pos(@x), #0) → #GT
#compare(#pos(@x), #neg(@y)) → #GT
#compare(#pos(@x), #pos(@y)) → #compare(@x, @y)
#compare(#s(@x), #0) → #GT
#compare(#s(@x), #s(@y)) → #compare(@x, @y)

Rewrite Strategy: INNERMOST

(1) DecreasingLoopProof (EQUIVALENT transformation)

The following loop(s) give(s) rise to the lower bound Ω(n1):
The rewrite sequence
append(::(@x4141_3, @xs4142_3), @l2) →+ ::(@x4141_3, append(@xs4142_3, @l2))
gives rise to a decreasing loop by considering the right hand sides subterm at position [1].
The pumping substitution is [@xs4142_3 / ::(@x4141_3, @xs4142_3)].
The result substitution is [ ].

(2) BOUNDS(n^1, INF)