koans

open class Tag(val name: String) {
    val children = mutableListOf<Tag>()
    val attributes = mutableListOf<Attribute>()

    override fun toString(): String {
        return "<$name" +
                (if (attributes.isEmpty()) "" else attributes.joinToString(separator = "", prefix = " ")) + ">" +
                (if (children.isEmpty()) "" else children.joinToString(separator = "")) +
                "</$name>"
    }
}

class Attribute(val name: String, val value: String) {
    override fun toString() = """$name="$value" """
}

fun <T : Tag> T.set(name: String, value: String?): T {
    if (value != null) {
        attributes.add(Attribute(name, value))
    }
    return this
}