How to build a package variant using another package variant as an input with 'guix build'?

[wakyct]

Hello all, I am new to Guix system and trying to build a package variant which depends on another package variant as 
an input, from the command line in a guix shell. I can build the variants using --with-git-url and --with-commit, but 
I can't figure out how to then add the input using --with-input.

For example, this is what I tried:

$ guix shell --container --nesting --network --development python-pivy soqt 
--no-grafts nss-certs
george@lucretius ~ [env]$ guix build soqt 
--with-git-url=soqt=https://github.com/coin3d/soqt \
> --with-commit=soqt=v1.6.2 --no-grafts --no-substitutes

updating checkout of 'https://github.com/coin3d/soqt'...
updating submodule 'build/general'...
updating submodule 'cpack.d'...
updating submodule 'data'...
updating submodule 'src/Inventor/Qt/common'...
retrieved commit 17364833049d3855f5e9e0f7908eb5ba08209318
/gnu/store/xjklc67c6bnmwjpryyx9b639vrhx2hjf-soqt-1.6.2

george@lucretius ~ [env]$ guix build python-pivy --with-git-url=python-pivy=https://github.com/coin3d/pivy \ 
> --with-commit=python-pivy=0.6.8 --with-input=soqt=soqt@1.6.2 --no-grafts 
> --no-substitutes

guix build: error: soqt: package not found for version 1.6.2


Probably this errors because soqt is not installed and/or in a channel? But as expected I can't install soqt 1.6.2 in 
this current shell container. Is there an ad hoc way to build variants using the results of previous builds in the 
same container? Or perhaps I need to do this in a different way? Thanks for any help. 


George