class Regexp::Lexer

def ascend(type, token)
  case type
  when :group, :assertion
    self.nesting = nesting - 1 if CLOSING_TOKENS.include?(token)
  when :set
    self.set_nesting = set_nesting - 1 if token == :close
  when :conditional
    self.conditional_nesting = conditional_nesting - 1 if token == :close
  end
end