General ideas : 

Various gf : Variable, Target, Opt
Notion of domain, and meshes. Precise concept in appendix.

- domains
- meshes
- scalar_valued and matrix_valued target.

- 1 page per gf ?
  - imfreq, etc... 
    in each page : 
      - include
      - synopsis
      - example
      - are they wrapped to python ? -> a cython example ?
      - singularity : documentation of local tail    
    
   - block : iterators. a bit more specific.

   - product gf.

- Common operations for all gf.

- creating using a factory. Very few constructors.
- regular vs view. rebind, assignment delegation
- evaluation
- grid access. The mesh point mechanism
- arithmetic
- lazy
- hdf5
- mpi
- slice.
- partial_evaluation and curry.
    examples.

- Operations on the gf 

  * Fourier transformation.
     notion of lazy
- implementation guide.
  The various traits and their concept.