### 12.13.7 The Cartesian equation of a geometric object in the plane: equation

See section 13.12.5 for Cartesian equations of three-dimensional objects.

The equation command takes as argument a geometric object.

equation returns the Cartesian equation for the object. (Note that x and y must be formal variables, they might need to be purged with purge(x) and purge(y)).
Input:

equation(line(-1,i))

Output:

y = x + 1