diff --git a/code/ysolve.f90 b/code/ysolve.f90
index ede43c3..d967215 100644
--- a/code/ysolve.f90
+++ b/code/ysolve.f90
@@ -90,15 +90,23 @@
 
      SUBROUTINE solve
        IF (reaction_type == "onestep") THEN
-         CALL solve_
+         CALL solve_ (fonestep)
        ELSE IF (reaction_type == "twostep") THEN
-         CALL solve_
+         CALL solve_ (fns)
        ELSE
-         CALL solve_
+         WRITE(*,*)'ERROR, UNSUPPORTED REACTION ', trim(reaction_type), '. STOPPING'
+         STOP
        END IF
      END SUBROUTINE solve
 
-     SUBROUTINE solve_
+     SUBROUTINE solve_ (rhs)
+
+      INTERFACE
+       SUBROUTINE rhs(r1,f)
+        REAL, INTENT(IN),  DIMENSION(:,:) :: r1
+        REAL, INTENT(OUT), DIMENSION(:,:) :: f
+       END SUBROUTINE rhs
+      END INTERFACE
 
       INTEGER :: i,j,k,savenum
       REAL :: pflame,pflold,delf=0.
@@ -127,14 +135,7 @@
 
        CALL SET_BC
 
-       if ( reaction_type == "onestep" ) then
-         CALL RK4(fonestep)
-       else if ( reaction_type == "twostep" ) then
-         CALL RK4(fns)
-       else
-           WRITE(*,*) 'ERROR, UNDEFINED REACTION TYPE ', reaction_type
-           stop
-       end if
+       CALL RK4(rhs)
 
        IF (t_uf.ge.dt_uf) THEN
          pflold=pflame