Represents text made of tagged parts, e.g. a diagnostic message in which types, identifiers and punctuation are classified, so that tooling is able to render them with colors. Text that carries no classification is represented as a single part tagged TextTag.Text, so that a plain string is always representable and Text is always equal to the original string. Two rich texts are equal when they read the same. Classification does not take part in equality, since the places that compare texts - such as deciding whether two types can be told apart in a message - are asking about what reaches the reader.