modifying (rather than fully replacing) a build phases

[John Kehayias]

Hi Guixers,

A question that is either relatively simple or else getting into the weeds a 
bit, that I came across in my proposed patch 
<https://issues.guix.gnu.org/64213> The general question is: how can I modify a 
build phase without replacing it completely? More specifically (as seen in the 
proposed patch) there is a build phase consisting of just an "apply invoke" 
call where I want to remove one of the arguments given. Is there a simple way 
to do this rather than just manually copying and deleting that string?

I would assume so, but I wasn't sure how to do it and couldn't quite grep any 
examples. Some light exploring in the guix repl shows me that package-arguments 
is a keyword list (not sure the proper terminology in Guile) with #:phases a 
big gexp. Essentially I want to modify that to remove a string. I suppose it is 
a question of manipulating a gexp directly. (I'm sure macros can do all this 
but there doesn't seem to be a need for that at this level, right?)

Thanks in advance!
John