class Nokogiri::CSS::XPathVisitor

def read_a_and_positive_b(values)
  op = values[2].strip
  if op == "+"
    a = values[0].to_i
    b = values[3].to_i
  elsif op == "-"
    a = values[0].to_i
    b = a - (values[3].to_i % a)
  else
    raise ArgumentError, "expected an+b node to have either + or - as the operator, but is #{op.inspect}"
  end
  [a, b]
end