YES

TRS:

  or(true(),y) -> true()
  or(x,true()) -> true()
  or(false(),false()) -> false()
  mem(x,nil()) -> false()
  mem(x,set(y)) -> =(x,y)
  mem(x,union(y,z)) -> or(mem(x,y),mem(x,z))

max/plus interpretations on N:

  or_A(x1,x2) = max{3, 2, 2}
  or#_A(x1,x2) = max{5, 1, 3}
  true_A = 3
  true#_A = 0
  false_A = 3
  false#_A = 4
  mem_A(x1,x2) = max{3, -1, 1}
  mem#_A(x1,x2) = max{5, 2 + x1, 3}
  nil_A = 1
  nil#_A = 5
  set_A(x1) = max{1, 1 + x1}
  set#_A(x1) = max{0, 0}
  =_A(x1,x2) = max{2, 2, -1}
  =#_A(x1,x2) = max{0, 0, 0}
  union_A(x1,x2) = max{1, 1 + x1, 1 + x2}
  union#_A(x1,x2) = max{0, 0, 0}

precedence:

  mem > or = nil = set = = = union > true = false