let k = 1
let X = map from local index (i,j) to global index k
for each quadrilateral i
  for each node j in quadrilateral i
    if (i,j) is coincident with a node (i',j')->k' in X
      assign index k' to (i,j)
    else
      assign index k to (i,j) in X
      k = k + 1