From http://ssb22.user.srcf.net/4dml
(also mirrored at http://ssb22.gitlab.io/4dml just in case)
The 4DML transformation utility can be used for the rapid prototyping of complex structural transformations on data. It was intended to convert musical, technical or analytical notations from one form to another, but it has also been used to process data for report generation and social science research. “Rapid prototyping” means you can experiment with new ways of structuring your input, or new types of computer output, by making comparatively small changes to your transform specifications. 4DML can process both tree-like and matrix-like structures (in any number of dimensions), and supports multiple independent structures over the same data.
There is an in-depth description of 4DML in my PhD thesis, which has been published as a technical report UCAM-CL-TR-591 (PDF format).
There is also a reference manual in this repository.