The Rd interface provides the abstraction of a readable stream, that is a source of characters. Rd is a class interface; a number of specific implementations exist, for example for reading from a Text, a file, or a terminal. (The Wr interface provides the complementary abstraction of a writable stream.)
TYPE T = REF;
A value of type T represents an instance of a reader. (The type Rd.T is actually equated to type Text.Reader.)