class Liquid::Parser

def expression
  token = @tokens[@p]
  case token[0]
  when :id
    str = consume
    str << variable_lookups
  when :open_square
    str = consume
    str << expression
    str << consume(:close_square)
    str << variable_lookups
  when :string, :number
    consume
  when :open_round
    consume
    first = expression
    consume(:dotdot)
    last = expression
    consume(:close_round)
    "(#{first}..#{last})"
  else
    raise SyntaxError, "#{token} is not a valid expression"
  end
end