class BinData::LazyEvaluator

def eval(obj, val, overrides = {})

+overrides+.
Lazily evaluates +val+ in the context of +obj+, with possibility of 
def eval(obj, val, overrides = {})
  if can_eval?(val)
    env = self.new(obj, overrides)
    env.lazy_eval(val)
  else
    val
  end
end