[eigen] Defining a custom procedurally defined matrix type (circular rotation of an existing matrix)

[Wenzel Jakob <wenzel@xxxxxxxxxxxxxxxxxxxx>]

Hi,

I'm wondering if it is possible to define a custom new matrix type in Eigen 
that is evaluated procedurally (i.e. not directly backed by an array)?

In particular I'm looking to define a new matrix type that's defined as the 
circular shift (in rows and columns) of another (dense) matrix. This means 
that an access to coeff() entails two modulo operations before evaluating 
the original matrix.

If there is some existing example of such an a procedurally defined matrix, 
it would be great if you could point me to it.

Thanks,
Wenzel