This part describes the implementation of Rubato Composer. A few words are in order about some design choices. The most important is obviously the programming environment. The predecessors presto and classic Rubato have been targeted at very specific platforms. In particular presto was implemented in C using the framework provided by the Atari ST. Classic Rubato was implemented in Objective C and required the framework of NEXTSTEP and, later, of MacOS X. The consequence of this commitment is that the software did not outlive the platform, i.e., Atari ST in the case of presto. In the case of classic Rubato, the software was limited to run on the platforms that supported the OpenSTEP framework, and no attempt to port to a different platform has been made. In both cases, this fixation proved to be a hindrance to a more widespread distribution and use of the software. It is to be noted that OpenMusic, which has been implemented in a Lisp system particular to the Macintosh platform, suffers from a similar drawback, although some effort to port OpenMusic to Linux has been made.