[Proj] Add simple transformations to proj

[Jan Hartmann]

Hi Frank and all, happy new year!

Is the following possible with Proj? I am working with historical maps 
that are rotated with respect to a well known epsg-projection, see for 
example

http://mapserver.sara.nl/a1000/bladgrenzen.html

The blue squares are the sheets of the map 1:1000 of Amsterdam after 
1940, in epsg:28992, the red squares are the same sheets before 1940 in 
a local coordinate system. I know the conversion coefficients for the 
two coordinate systems: a simple six-value transformation of the form:

xnew = a*xold + b*yold + c
ynew = d*xold + e*yold + f

I would like to access the second map via a proj-string. For that, I 
would have to expand proj with a procedure that takes the six 
coefficients (a-f) and generates the transformed coordinates after the 
regular transformation.

Is this possible? Can anyone give me some pointers where to look in the 
proj-code? Conceptually, it looks simple to me, but I don't know where 
to start.

Jan
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