Correct way to pull using subtree merge without guesswork?

Hello,

can someone please tell me, if this is the correct syntax for using "git pull" to pull in "subtree merging strategy", if I don't want to allow git to guess where to do the changes:

git pull -s subtree -X subtree=path/to/subtree externalproject master

Returns with success, but is "-X subtree=" the right way to force a subtree path and disallow git to autodetect the subtree?

$ git --version git version 1.7.4.4

Thank you very much in advance

Yours

Manuel