[dune-functions] HierarchicalLagrangeBasis extension space

[Lasse Hinrichsen-Bischoff]

Dear dune-functions community,

I'm trying to implement a preconditioned hierarchical error estimator.
My ansatz space is P1 and I want to estimate the error in Q=P2 such that 
I have Q = S + V, where S = P1 and V are the quadratic bubble functions.

It seems the HierarchicalLagrangeBasis is what I would want to use.

My question is, how do I know which indices correspond to the quadratic 
elements and which are piecewise linear? The only thing I came up so far 
is looking into the code of the local FE and noticing that the local 
indices 0, 2 and 5 correspond to the linear parts and 1, 3 and 4 to the 
quadratic ones (in 2D). Is it okay to use this fact and construct index 
mappings from here?

Best,
Lasse