class Regexp::Expression::Sequence

def add_to(subexpression, params = {}, active_opts = {})
  sequence = at_levels(
    subexpression.level,
    subexpression.set_level,
    params[:conditional_level] || subexpression.conditional_level
  )
  sequence.nesting_level = subexpression.nesting_level + 1
  sequence.options = active_opts
  subexpression.expressions << sequence
  sequence
end