capabilities of the native windowing system happen to be. A graphic version of Stella is currently being developed for the Macintosh. When completed, it will enhance, rather than replace the current command interface; it will always be possible to specify actions and commands via a command interpreter as well as by mouse and menu.

By using Stella, a composer is free to work on different sections of music in parallel, sometimes listening to material, sometimes analyzing it, sometimes editing sound parameters or running algorithms whatever is most appropriate for that particular point in the compositional process.

One of the more powerful features of Stella is its ability to select, or reference, not only existing structure, but virtual subsets from this structure as well. This capability is a requirement for any editor that would attempt to support a process as complicated as sound event editing. To be useful, a composition editor must not only be able to edit existing structure, but also non-adjacent subsets from this material as well. Since composers are as interested in changes between sound events as within them, it must also be possible to reference clusters of events in parallel such that concurrent comparative analysis can be performed.

The typical case for referencing virtual subset occurs when selecting sub-objects of a Container. The editor currently distinguishes between five basic types of sub-object reference: index, a single object; range, all the objects between a low and high bound; iteration, non-adjacent objects between a low and high bound; grouping, clusters of objects between a low and high bound; sequence, combinations of all the above. To notate sub-objects a simple scheme is used in which a Container reference (absolute or positional) is qualified with a sub-object specification denoted inside brackets.

For example, examp[1] selects the first object in a container named Examp, and examp[10:20:2] selects every other element between the tenth and twentieth positions in Examp. The full-blown notation of a sub-object reference is four fields in length, with fields delimited by colons: low:high:step:width, where low is the low bound of the sub-object reference, high is the inclusive upper bound, step is the stepping increment between objects in the reference and width is the number of objects to reference in parallel. It is only necessary to specify enough of the fields to adequately describe the reference.

Fields may also contain 'wildcards', and may be elided if a field default applies. For example, the * character references all of the objects in a container, no matter how many there are. An entire reference may itself be delimited by comma if there are more than one of them (Figure 3).