anchor.diff:              tlen = sqrt(taup(1)**2 + taup(2)**2 + taup(3)**2)
   
               DO m = 1,3
  -               xtangents(i,j,m) = taup(m)
  +               xtangents(i,j,m) = taup(m)/tlen
               ENDDO
               edge_lengths(i,j,1) = tlen
            ENDDO
  
positive.code: 
  c           # nv has unit length
              do m = 1,3
                 ytangents(i,j,m) = taup(m)
              enddo
              edge_lengths(i,j,2) = tlen
           enddo
  
negative.code:          ENDDO
        ENDDO
  
        DO j = 1-mbc,my+mbc+1
           DO i =  -mbc,mx+mbc+1
  c           # Now do y-faces
  