function triangle(x,y,g,zero,xb,yb,xe,ye)
*              ========
*     Contourlines in a triangle
*     --------------------------
* ... Coordinates and function values
*     at triangle vertices:
      real x(3),y(3),f(3),g(3)
* ... Permutations & actions:
      integer np(3,6),must(27),move(27)
      logical triangle,first
*
* ... Save the following:
      data np,must,move / 18*0 , 27*0 , 27*0 /
      data first / .true. /
* ... Initialize actions & permutations:
      if(first) then
        call setup3(np,must,move)
        first=.false.
      endif
*
* ... Case encoding (base 3):
      more=0
      do 10 k=1,3
	f(k)=g(k)-zero
        if(f(k).eq.0.) more=more*3
        if(f(k).lt.0.) more=more*3+1
        if(f(k).gt.0.) more=more*3+2
10    continue
      label=must(more+1)
*
* ... Permutation of (1,2,3) case:
      keus=move(more+1)
      x1=x(np(1,keus))
      x2=x(np(2,keus))
      x3=x(np(3,keus))
      y1=y(np(1,keus))
      y2=y(np(2,keus))
      y3=y(np(3,keus))
      f1=f(np(1,keus))
      f2=f(np(2,keus))
      f3=f(np(3,keus))
      xb=x1
      xe=x2
      yb=y1
      ye=y2
*
* ... Perform action:
      triangle=.false.
      goto (1,2,3,4,5,6),label
*
* ... Nothing:
1     continue
* ... ===
3     continue
* ... =<<
5     continue
* ... <<<
      return
*
* ... Line through one side:
2     continue
* ... ==<
      triangle=.true.
      return
*
* ... Line through one vertex:
4     continue
* ... =<>
      xe=(f2*x3-f3*x2)/(f2-f3)
      ye=(f2*y3-f3*y2)/(f2-f3)
      triangle=.true.
      return
*
* ... Line through two sides:
6     continue
* ... <<>
      xb=(f1*x3-f3*x1)/(f1-f3)
      yb=(f1*y3-f3*y1)/(f1-f3)
      xe=(f2*x3-f3*x2)/(f2-f3)
      ye=(f2*y3-f3*y2)/(f2-f3)
      triangle=.true.
      return
*
      end