class Phlex::SGML

def __classes__(c)
	case c
	when String
		c
	when Symbol
		c.name.tr("_", "-")
	when Array, Set
		c.filter_map { |c| __classes__(c) }.join(" ")
	when Hash
		c.filter_map { |c, add|
			next unless add
			case c
				when String then c
				when Symbol then c.name.tr("_", "-").delete_suffix("?")
				else raise Phlex::ArgumentError.new("Class keys should be Strings or Symbols.")
			end
		}.join(" ")
	when nil, false
		nil
	else
		if c.respond_to?(:to_phlex_attribute_value)
			c.to_phlex_attribute_value
		elsif c.respond_to?(:to_str)
			c.to_str
		else
			c.to_s
		end
	end
end