Fish shell support problem (with virtualenv)

[proItheus]

When I use . nodeenv/bin/activate.fish in a python virtual env, I get the following error:

functions: Function '_old_fish_prompt' already exists. Cannot create copy 'fish_prompt'

nodeenv/bin/activate.fish (line 115): 
    functions -c fish_prompt _old_fish_prompt
    ^
from sourcing file env/bin/activate.fish

and I find that in bash's activate script, you use OLD_NODE_VIRTUAL_PS1 to avoid conflict with virtualenv.Hope you can do this for fish too.