def find_by_identity(identity, at_date=nil)
      # (at_date.nil? ? current : at(at_date)).where(IDENTITY_COLUMN=>identity).first
      if at_date.nil?
        q = current
      else
        q = at(at_date)
      end
      q = q.where(IDENTITY_COLUMN=>identity)
      q.first
    end
def find_by_identity(identity, at_date=nil)
      # (at_date.nil? ? current : at(at_date)).where(IDENTITY_COLUMN=>identity).first
      if at_date.nil?
        q = current
      else
        q = at(at_date)
      end
      q = q.where(IDENTITY_COLUMN=>identity)
      q.first
    end
Note that find_by_identity will return nil if there's not a current iteration of the identity