$ nix-shell
these derivations will be built:
  /nix/store/3gg9b0pdwxjnq94si973fn6sifdh4qx8-python-2.7.15-env.drv
building '/nix/store/3gg9b0pdwxjnq94si973fn6sifdh4qx8-python-2.7.15-env.drv'...
collision between `/nix/store/jlqy9ibk9iskjsr1blw5gfd73dppjkfn-python2.7-tensorflow-tensorboard-1.5.1/bin/.tensorboard-wrapped' and `/nix/store/r0rvr0sr8z7dalm3sz7af0x1iyw0fa8i-python2.7-tensorflow-1.5.0/bin/.tensorboard-wrapped'
builder for '/nix/store/3gg9b0pdwxjnq94si973fn6sifdh4qx8-python-2.7.15-env.drv' failed with exit code 25
error: build of '/nix/store/3gg9b0pdwxjnq94si973fn6sifdh4qx8-python-2.7.15-env.drv' failed