[R] gbm package: relationship between interaction.depth and number of features?

[Tim Converse]

I'm puzzled by the interaction.depth argument to the gbm() function, and
whether it specifies just the tree depth, or also (implicitly) the number of
distinct features used in splits.

>From some version of the documentation I have:

"interaction.depth     The maximum depth of variable interactions. 1 implies
an additive model, 2 implies a model with up to 2-way interactions, etc."

In practice it seems that an interaction.depth of 2 produces trees of depth
2 (i.e. four terminal leaves), but we have never seen any such trees using
more than two features, even though there are three non-leaf nodes available
to split on.  

Is it possible for gbm to produce a depth-two tree using three distinct
features (at the root, the left subtree, and right subtree)?  Or does the
algorithm impose a constraint that all splits at a given depth must be on
the same feature?

thanks,
  Tim