For example in the paper-eng, if you have a backend that doesn't fully
support a symcache (yet) it's handy to be able to ignore processing
other paper-eng txns when all you care about at the moment is the
simulated symbol.
NOTE, that currently this will still result in a key-error when you load
more then one mkt with the paper engine (for which the backend does not
have the symcache implemented) since no fqme ad-hoc query was made for
the 2nd symbol (and i'm not sure we should support that kinda hackery
over just encouraging the sym-cache being added?). Def needs a little
more thought depending on how many backends are never going to be able
to (easily) support caching..
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