# Implementation of the moldule {front}.
# Last edited on 2021-02-08 19:53:02 by jstolfi

def get_one(R, rev):
  n = len(R)
  for k in range(n):
    elk = R[k] if not rev else R[n-1-k]
    if elk.htini != None: 
      return [ elk ]
  return []

def get_initial_fringe (R):
  L = []
  for elk in R:
    ctk = path.contacts(elk)
    if (len(ctk) == 0) and (elk.htini == None):
      L.append(elk)
  return L

def get_initial_all(R):
  L = []
  for elk in R:
    if (elk.htini == None):
      L.append(elk)
  return L

def add_element(L, el):
  if el not in L and el.htini != None:
    L.append(el_new)
    return True
  else:
    return False