anchor.diff:  
     410 continue
   c     # zero-order extrapolation:
  -      do 415 m=1,meqn
  -         do 415 jbc=1,mbc
  -            do 415 i = 1-mbc,mx+mbc
  +      do m=1,meqn
  +         do jbc=1,mbc
  +            do i = 1-mbc,mx+mbc
                  q(i,my+jbc,m) = q(i,my,m)
  -  415       continue
  +            end do
  +         end do
  +      end do
         go to 499
   
     420 continue
   c     # periodic:
  -      do 425 m=1,meqn
  -         do 425 jbc=1,mbc
  -            do 425 i = 1-mbc, mx+mbc
  -               q(i,my+jbc,m) = q(i,jbc,m)
  -  425       continue
  +      write(6,*) 'clawpack4.6_bc2_default : ', 
  +     &            'Periodic BCs should not be set in bc2'
  +      stop
         go to 499
   
     430 continue
   c     # solid wall (assumes 3'rd component is velocity or momentum in y):
  -      do 435 m=1,meqn
  -         do 435 jbc=1,mbc
  -            do 435 i = 1-mbc, mx+mbc
  +      do m=1,meqn
  +         do jbc=1,mbc
  +            do i = 1-mbc, mx+mbc
                  q(i,my+jbc,m) = q(i,my+1-jbc,m)
  -  435       continue
  -c     # negate the normal velocity:
  -      do 436 jbc=1,mbc
  -         do 436 i = 1-mbc, mx+mbc
  +            end do
  +         end do
  +      end do
  +c     # negate the normal velocity:
  +      do jbc=1,mbc
  +         do i = 1-mbc, mx+mbc
               q(i,my+jbc,3) = -q(i,my+1-jbc,3)
  -  436    continue
  +         end do
  +      end do
         go to 499
   
     499 continue
  
positive.code: c
    310 continue
  c     # zero-order extrapolation:
        do 315 m=1,meqn
           do 315 jbc=1,mbc
              do 315 i = 1-mbc,mx+mbc
                 q(i,1-jbc,m) = q(i,1,m)
    315       continue
        go to 399
  
    320 continue
  c     # periodic:
        do 325 m=1,meqn
           do 325 jbc=1,mbc
              do 325 i = 1-mbc, mx+mbc
                 q(i,1-jbc,m) = q(i,my+1-jbc,m)
    325       continue
        go to 399
  
    330 continue
  c     # solid wall (assumes 3'rd component is velocity or momentum in y):
        do 335 m=1,meqn
           do 335 jbc=1,mbc
              do 335 i = 1-mbc,mx+mbc
                 q(i,1-jbc,m) = q(i,jbc,m)
    335       continue
  c     # negate the normal velocity:
        do 336 jbc=1,mbc
           do 336 i = 1-mbc, mx+mbc
              q(i,1-jbc,3) = -q(i,jbc,3)
    336    continue
        go to 399
  
    399 continue
  
negative.code: 
    210 continue
  c     # zero-order extrapolation:
        do m=1,meqn
           do ibc=1,mbc
              do j = 1-mbc,my+mbc
                 q(mx+ibc,j,m) = q(mx,j,m)
              end do
           end do
        end do
        go to 299
  
    220 continue
  c     # periodic:
        write(6,*) 'clawpack4.6_bc2_default : ', 
       &            'Periodic BCs should not be set in bc2'
        stop
        go to 299
  
    230 continue
  c     # solid wall (assumes 2'nd component is velocity or momentum in x):
        do m=1,meqn
           do ibc=1,mbc
              do j = 1-mbc, my+mbc
                 q(mx+ibc,j,m) = q(mx+1-ibc,j,m)
              end do
           end do
        end do
  c     # negate the normal velocity:
        do ibc=1,mbc
           do j = 1-mbc, my+mbc
              q(mx+ibc,j,2) = -q(mx+1-ibc,j,2)
           end do
        end do
        go to 299
  
    299 continue
  