Also - is there a complete guide on writing varexp and selection strings? In particular the hidden iteration variable seems to work different depending on whether you explicitly use empty square [] brackets? How can I specify Phep[][1] without doing this?
I’m not interested in solutions that involve copying to arrays.
The explanation in the TTree::Draw documentation should contain the information you are looking for.
There should be no difference between idhep and idhep[] but Phep[][1] and Phep[1] (or Phep[1][]) are completely different (The first one fix the value of the 2nd index while the second one fix the value of the 2nd index while iterating through a 2 dimension array).