We describe the design and implementation of a Standard ML of New Jersey library for the interactive pretty-printing of possibly infinite syntax trees. The library handles elision in a novel way, and is implemented using Concurrent ML and the eXene X Window System toolkit. 1 Pretty-printing with Elision In the modern approach to pretty-printing, as developed by Oppen [Opp80] and others [Mik81, MCC86, Hug95, Wad98], and featured in Paulson's textbook [Pau96], prettyof abstract syntax is a two-stage process: an abstract syntax tree is converted into a pretty-printing tree that abstractly describes how the syntax tree should be formatted. Pretty-printing trees are made up of strings, potential line-break points, and various kinds of blocks [Opp80, Pau96] or boxes [Mik81, MCC86, Hug95, Wad98].