class Aws::S3::PresignedPost

def self.define_field(field, *args, &block)
  @@allowed_fields << field
  options = args.last.is_a?(Hash) ? args.pop : {}
  field_name = args.last || field.to_s
  if block_given?
    define_method("#{field}", block)
  else
    define_method("#{field}") do |value|
      with(field_name, value)
    end
    if options[:starts_with]
      @@allowed_fields << "#{field}_starts_with".to_sym
      define_method("#{field}_starts_with") do |value|
        starts_with(field_name, value)
      end
    end
  end
end