This namespace defines a set of classes and functions to build and evaluate symbolic expressions of scalar type Index. Here is a simple example:

// First step, defines symbols:
struct x_tag {};  static const symbolic::SymbolExpr<x_tag> x;
struct y_tag {};  static const symbolic::SymbolExpr<y_tag> y;
struct z_tag {};  static const symbolic::SymbolExpr<z_tag> z;
 
// Defines an expression:
auto expr = (x+3)/y+z;
 
// And evaluate it: (c++14)
std::cout << expr.eval(x=6,y=3,z=-13) << "\n";
 
// In c++98/11, only one symbol per expression is supported for now:
auto expr98 = (3-x)/2;
std::cout << expr98.eval(x=6) << "\n";

It is currently only used internally to define and manipulate the Eigen::last and Eigen::lastp1 symbols in Eigen::seq and Eigen::seqN.