Is principle of constancy of light velovity necessary?

On Jul 8, 4:26*pm, Tom Roberts wrote in
sci.physics.relativity:
But yes, in GR (not SR) light does not always propagate with speed c,
when measured in vacuum over non-local paths.

Bravo Honest Roberts but you should start interpreting this
variability of the speed of light QUANTITATIVELY: How does the speed
of light vary with the gravitational potential V? Einstein's 1911
equation c'=c(1+V/c^2)? If this equation is wrong Honest Roberts, as
you and your brother zombie Paul Andersen often suggest, then why
don't you give the correct equation? Finally, does the frequency shift
f'=f(1+V/c^2) experimentally confirmed by Pound and Rebka have
anything to do with Einstein's 1911 equation c'=c(1+V/c^2) or,
generally, with the fact that the speed of light varies with position
in a gravitational field?

These are important questions Honest Roberts.....

Pentcho Valev