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:

  lexicographic combination of reduction pairs:
  
    1. weighted path order
    
      base order:
    
        max/plus interpretations on natural numbers:
        
          lessleaves#_A(x1,x2) = max{x1 + 2, x2 + 4}
          cons_A(x1,x2) = max{x1 + 6, x2}
          concat_A(x1,x2) = max{x1 + 6, x2}
          leaf_A = 0
    
      precedence:
      
        lessleaves# > cons = concat = leaf
      
      partial status:
    
        pi(lessleaves#) = [2]
        pi(cons) = [1, 2]
        pi(concat) = [1, 2]
        pi(leaf) = []
    
    2. weighted path order
    
      base order:
    
        max/plus interpretations on natural numbers:
        
          lessleaves#_A(x1,x2) = max{13, x2 + 5}
          cons_A(x1,x2) = max{8, x1 + 3}
          concat_A(x1,x2) = max{7, x1}
          leaf_A = 0
    
      precedence:
      
        cons = concat > lessleaves# = leaf
      
      partial status:
    
        pi(lessleaves#) = [2]
        pi(cons) = [1]
        pi(concat) = []
        pi(leaf) = []
    

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:

  lexicographic combination of reduction pairs:
  
    1. weighted path order
    
      base order:
    
        max/plus interpretations on natural numbers:
        
          concat#_A(x1,x2) = x1 + 1
          cons_A(x1,x2) = max{x1, x2 + 1}
    
      precedence:
      
        concat# = cons
      
      partial status:
    
        pi(concat#) = [1]
        pi(cons) = [1, 2]
    
    2. weighted path order
    
      base order:
    
        max/plus interpretations on natural numbers:
        
          concat#_A(x1,x2) = max{0, x1 - 1}
          cons_A(x1,x2) = max{x1, x2}
    
      precedence:
      
        concat# = cons
      
      partial status:
    
        pi(concat#) = []
        pi(cons) = [1, 2]
    

The next rules are strictly ordered:

  p1

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