let us consider the equations

x^2 -c^2r^2 =0

x' ^2 - c' ^2 t' ^2 =0

SR deals with the situation of connecting x,t,c with x', t', c'

Now let us suppose that light speed varies such that c does not equal c', say:

c' = c(1-v^2/c^2)

where v is a general velocity

we don't have SR.

But lets try v instead of being a general velocity as due to gravity v^2 = 2GM/r

then we have

dr/dt = (1 - 2*G*M/r/c^2)*c.

But that's GR.

Observation: wow, that's a bit weird we don't get GR from SR.

I wonder what kind of fantasy land could we go into, as we sit with Einstein on his light beam in his thought experiment. Well maybe we could think c' is not dependent on v some days like Monday, Tuesday and Wednesdays, but on Sundays it is; seems to make as much sense as anything else in Einstein's cuckooland.
c. RJAnderton2009

Now I got a proof, now I got it, hiphip hooray ...