      subroutine mdm
     *     (vk,tail, v,l, last,next, mark)
clll. optimize
c***********************************************************************
c  mdm -- form element from uneliminated neighbors of vk
c***********************************************************************
      integer  vk, tail,  v(1), l(1),   last(1), next(1),   mark(1),
     *   tag, s,ls,vs,es, b,lb,vb, blp,blpmax
      equivalence  (vs, es)
c
c----initialize tag and list of uneliminated neighbors
      tag = mark(vk)
      tail = vk
c
c----for each vertex/element vs/es in element list of vk
      ls = l(vk)
   1  s = ls
      if (s.eq.0)  go to 5
        ls = l(s)
        vs = v(s)
        if (next(vs).lt.0)  go to 2
c
c------if vs is uneliminated vertex, then tag and append to list of
c------uneliminated neighbors
          mark(vs) = tag
          l(tail) = s
          tail = s
          go to 4
c
c------if es is active element, then ...
c--------for each vertex vb in boundary list of element es
   2      lb = l(es)
          blpmax = last(es)
          do 3 blp=1,blpmax
            b = lb
            lb = l(b)
            vb = v(b)
c
c----------if vb is untagged vertex, then tag and append to list of
c----------uneliminated neighbors
            if (mark(vb).ge.tag)  go to 3
              mark(vb) = tag
              l(tail) = b
              tail = b
   3        continue
c
c--------mark es inactive
          mark(es) = tag
c
   4    go to 1
c
c----terminate list of uneliminated neighbors
   5  l(tail) = 0
c
      return
      end
