Lean4
@[simps]
instance comp : (F ⋙ G).LaxMonoidal where
ε := ε G ≫ G.map (ε F)
μ X Y := μ G _ _ ≫ G.map (μ F X Y)
μ_natural_left _ _ := by simp_rw [comp_obj, F.comp_map, μ_natural_left_assoc, assoc, ← G.map_comp, μ_natural_left]
μ_natural_right _ _ := by simp_rw [comp_obj, F.comp_map, μ_natural_right_assoc, assoc, ← G.map_comp, μ_natural_right]
associativity _ _
_ := by
dsimp
simp_rw [comp_whiskerRight, assoc, μ_natural_left_assoc, MonoidalCategory.whiskerLeft_comp, assoc,
μ_natural_right_assoc, ← associativity_assoc, ← G.map_comp, associativity]