def horizon_float(k0, plus_or_minus):
   """Returns a floating point representation of k direction face index; eg. 3.5 for face between layers 3 and 4."""
   result = float(k0)
   if plus_or_minus == '+': result += 0.5
   elif plus_or_minus == '-': result -= 0.5
   else: assert False
   return result