YES

We show the termination of the TRS R:

  concat(leaf(),Y) -> Y
  concat(cons(U,V),Y) -> cons(U,concat(V,Y))
  lessleaves(X,leaf()) -> false()
  lessleaves(leaf(),cons(W,Z)) -> true()
  lessleaves(cons(U,V),cons(W,Z)) -> lessleaves(concat(U,V),concat(W,Z))

-- SCC decomposition.

Consider the dependency pair problem (P, R), where P consists of

p1: concat#(cons(U,V),Y) -> concat#(V,Y)
p2: lessleaves#(cons(U,V),cons(W,Z)) -> lessleaves#(concat(U,V),concat(W,Z))
p3: lessleaves#(cons(U,V),cons(W,Z)) -> concat#(U,V)
p4: lessleaves#(cons(U,V),cons(W,Z)) -> concat#(W,Z)

and R consists of:

r1: concat(leaf(),Y) -> Y
r2: concat(cons(U,V),Y) -> cons(U,concat(V,Y))
r3: lessleaves(X,leaf()) -> false()
r4: lessleaves(leaf(),cons(W,Z)) -> true()
r5: lessleaves(cons(U,V),cons(W,Z)) -> lessleaves(concat(U,V),concat(W,Z))

The estimated dependency graph contains the following SCCs:

  {p2}
  {p1}


-- Reduction pair.

Consider the dependency pair problem (P, R), where P consists of

p1: lessleaves#(cons(U,V),cons(W,Z)) -> lessleaves#(concat(U,V),concat(W,Z))

and R consists of:

r1: concat(leaf(),Y) -> Y
r2: concat(cons(U,V),Y) -> cons(U,concat(V,Y))
r3: lessleaves(X,leaf()) -> false()
r4: lessleaves(leaf(),cons(W,Z)) -> true()
r5: lessleaves(cons(U,V),cons(W,Z)) -> lessleaves(concat(U,V),concat(W,Z))

The set of usable rules consists of

  r1, r2

Take the reduction pair:

  matrix interpretations:
  
    carrier: N^1
    order: standard order
    interpretations:
      lessleaves#_A(x1,x2) = x2
      cons_A(x1,x2) = x1 + x2 + 1
      concat_A(x1,x2) = x1 + x2
      leaf_A() = 0

The next rules are strictly ordered:

  p1

We remove them from the problem.  Then no dependency pair remains.

-- Reduction pair.

Consider the dependency pair problem (P, R), where P consists of

p1: concat#(cons(U,V),Y) -> concat#(V,Y)

and R consists of:

r1: concat(leaf(),Y) -> Y
r2: concat(cons(U,V),Y) -> cons(U,concat(V,Y))
r3: lessleaves(X,leaf()) -> false()
r4: lessleaves(leaf(),cons(W,Z)) -> true()
r5: lessleaves(cons(U,V),cons(W,Z)) -> lessleaves(concat(U,V),concat(W,Z))

The set of usable rules consists of

  (no rules)

Take the reduction pair:

  matrix interpretations:
  
    carrier: N^1
    order: standard order
    interpretations:
      concat#_A(x1,x2) = x1
      cons_A(x1,x2) = x2 + 1

The next rules are strictly ordered:

  p1

We remove them from the problem.  Then no dependency pair remains.