diff --git a/src/matcha/subdomain_s.f90 b/src/matcha/subdomain_s.f90
index ac68f62..f5bfa0c 100644
--- a/src/matcha/subdomain_s.f90
+++ b/src/matcha/subdomain_s.f90
@@ -59,6 +59,16 @@
     my_dz = dz_
   end procedure
 
+  module procedure add
+    call assert(allocated(lhs%s_) .and. allocated(rhs%s_), "subdomain_t%add: allocated(rhs%s_)")
+    total%s_ = lhs%s_ +  rhs%s_
+  end procedure
+
+  module procedure multiply
+    call assert(allocated(rhs%s_), "subdomain_t%multiply: allocated(rhs%s_)")
+    product%s_ = lhs +  rhs%s_
+  end procedure
+
   module procedure assign_
     call assert(allocated(rhs%s_), "subdomain_t%assign_: allocated(rhs%s_)")
     lhs%s_ =  rhs%s_
@@ -142,7 +152,6 @@ subroutine apply_boundary_condition(ds)
       if (me==num_subdomains) ds(my_nx,:,:) = 0.
     end subroutine
 
-
   end procedure
 
   pure module function laplacian(rhs) result(laplacian_rhs)