Index: README.md
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--- README.md
+++ README.md
@@ -1,17 +1,25 @@
 # **GRG**
 
 ## Computer Algebra System for Differential Geometry, Gravitation and Field Theory
 
+----
+
+## Introduction
+
 The computer algebra system **GRG** is designed to make calculation in differential geometry and field theory as simple and natural as possible. **GRG** is based on the computer algebra system **REDUCE** but **GRG** has its own simple input language whose commands resemble short English phrases.
 
 **GRG** understands tensors, spinors, vectors, differential forms and knows all standard operations with these quantities. Input form for mathematical expressions is very close to traditional mathematical notation including Einstein summation rule. **GRG** knows covariant properties of the objects: one can easily raise and lower indices, compute covariant and Lie derivatives, perform coordinate and frame transformations etc. **GRG** works in any dimension and allows one to represent tensor quantities with respect to holonomic, orthogonal and even any other arbitrary frame.
 
 One of the key features of **GRG** is that it knows a large number of built-in usual field-theoretical and geometrical quantities and formulas for their computation providing ready solutions to many standard problems.
 
 Another unique feature of **GRG** is that it can export results of calculations into other computer algebra system such as *Maple*, *Mathematica*, *Macsyma* or ***REDUCE*** in order to use these systems to proceed with analysis of the data. The *LaTeX* output format is supported as well. **GRG** is compatible with the **REDUCE** graphics shells providing nice book-quality output with Greek letters, integral signs, etc.
 
+----
+
+## Features
+
 The main built-in **GRG** capabilities are:
 
 - Connection, torsion and nonmetricity.
 - Curvature.
 - Spinorial formalism.
@@ -25,14 +33,27 @@
 - Null congruences and optical scalars.
 - Kinematics for time-like congruences.
 - Ideal and spin fluid.
 - Newman-Penrose formalism.
 - Gravitational equations for the theory with arbitrary gravitational Lagrangian in Riemann and Riemann-Cartan spaces.
+
+----
+
+## Availability
+
+- [**GRG Homepage**](https://reduce-algebra.sourceforge.io/grg32/grg32.php)
+- [GitHub Mirror](https://github.com/reduce-algebra/grg/)
+- [SourceHut Mirror](https://git.sr.ht/~trn/grg/)
+- [Chisel Mirror](https://chiselapp.com/user/reduce-algebra/repository/grg/)
+
+----
 
 ## Documentation
 
 - [User Manual and Reference Guide](https://github.com/reduce-algebra/grg/tree/master/doc)
+
+----
 
 ## Author
 
 ```text
 Vadim V. Zhytnikov
@@ -45,13 +66,19 @@
 E-mail: vvzhy@td.lpi.ac.ru
 
 E-mail: grg@curie.phy.ncu.edu.tw
         Subject: for Zhytnikov
 ```
+
+----
 
 ## License
 
 - **GRG** is free of charge. See [LICENSE](https://github.com/reduce-algebra/grg/blob/master/LICENSE) for full details.
 
-## Upstream
+----
+
+## Homepage
 
 - [GRG Homepage](https://reduce-algebra.sourceforge.io/grg32/grg32.php)
+
+----