+
+(* Advanced constructions with nplus ****************************************)
+
+lemma depth_plus (p1) (p2):
+ āp2ā+āp1ā = āp1āp2ā.
+#p1 elim p1 -p1 //
+* [ #n ] #p1 #IH #p2 <list_append_lcons_sn
+[ <depth_d_sn <depth_d_sn //
+| <depth_m_sn <depth_m_sn //
+| <depth_L_sn <depth_L_sn //
+| <depth_A_sn <depth_A_sn //
+| <depth_S_sn <depth_S_sn //
+]
+qed.