sequence PMap issue



hello experts,

as you know sequence field may need a pmap if any of its children needs pmap for encoding. let’s assume program detects the current sequence field needs pmap for decoding.
sequence also contains length field (assume it needs pmap bit), I am wondering if i should decode a pmap before decoding length by using this new decoded pmap, or decode length field by using previous pmap then decode a pmap for other fields under sequence.

this information seems not being mentioned in the fast specification.

please advise

thanks & regards