This is a package originally made by Greg Nelson, to make it easy to pretty print parse trees. The documentation is in the interface. Here is a duplication of the introductory paragraph:

Formatted output routines. Send a stream of characters through a buffer, delineate the stream into subsequences called objects. The objects can be nested. (Objects are called "groups" in CGN9.) For each object, specify within it a set of breakpoints. The routines will format the text as follows. (1) An object will be printed all on one line if possible. (2) If an object cannot fit on one line, new lines will be started at one or more of the object's breakpoints, so that no text overflows the right margin. (3) With each breakpoint there is associated an integer call the breakpoint's offset. If a new line is started at a breakpoint, then the new line will be indented by the sum of the breakpoint's offset plus the indentation of the first character of the smallest object that contains the breakpoint. (4) Each breakpoint is either united or ununited. If any breakpoint of an object is broken, then all that object's united breakpoints are broken. An ununited breakpoint is broken only if it is necessary to do so to prevent overflowing the right margin.

StructuredStreams provides an IO.STREAM style interface to UnparserBuffer. Again, documented in the interface. Here's the introductory paragraph:

Greg Nelson has implemented a package called UnparserBuffer for his use in formatting Juno programs. StructuredStreams is a streams interface to it. A StructuredStream is an output stream, layered on another stream which recieves the formatted characters. A StructuredStream takes in a stream of characters, delimited into nested objects, each of which has a beginning, an end, and some breakpoints. The StructuredStream delivers to its understream the characters, interspersed with various white space characters (carriage returns and spaces) according to the formatting rules (described in UnparserBuffer.Mesa). The rules are such that characters sent to a StructuredStream outside of any object delimiters will not get any white space inserted among them. Also, StructuredStreams has been defined so that an ordinary stream used in place of a StructuredStream gives no ill effects.