Project without Clear?

I would like to call TTree::Project without first Clearing the histogram (TH1F). Is there an option which bypasses the call to Clear? I’m working in 4.03/4.
In general, what’s the best way to find the set of valid options for a root function call?

thanks, Dave

The TTree::Project arguments are described in TTree::Draw.
In your case do

tree.Project("myhist","myvar","myselection"); tree.Project("+myhist","myvar","mynewselection");