subroutine brick(xb,yb,zb,fb,zero)
*                =====
*     Isosurfaces in a brick
*     ----------------------
      real xb(8),yb(8),zb(8),fb(8)
      real xt(4),yt(4),zt(4),ft(4)
*
* ... Subdvision in tetrahedra:
      integer nt(4,5)
      data nt / 1,2,3,5 , 4,3,2,8 , 6,8,5,2 , 7,5,8,3 , 2,3,5,8 /
*
* ... Loop through tetrahedra:
      do 45 m=1,5
* ... Loop through vertices:
      do 50 n=1,4
      nm=nt(n,m)
      xt(n)=xb(nm)
      yt(n)=yb(nm)
      zt(n)=zb(nm)
      ft(n)=fb(nm)
50    continue
* ... Basic isosurfaces:
      call tetraedr(xt,yt,zt,ft,zero)
45    continue
      return
      end