It is here 😎.
To quote its Github page (link here):
SigmaEngine is a desktop mathematical notebook for interactive numerical work, symbolic differentiation, and C++ code generation.
It is built around the Minimal Math Library (MML): you type expressions, define functions, work with vectors and matrices, run numerical analysis operations, and keep a live generated C++ preview of the session.
The source project is developed privately. This public repository is the release home for downloadable installers, checksums, release notes, screenshots, and user-facing documentation.
What it can do is best shown with some screenshots:
Basic calculations.
Working with vectors.
Working with matrices.
