@get_number(tr, spec) for spec which has already been instantiated

[georgematheos]

This causes an error if tr is a TraceToken.
E.g.

# within kernel function...
set = @objects(tr, Blip)
# ... later
@get_number(tr, Blip(Aircraft(1), Timestep(2)))
# This causes an error because the number statement was looked up when constructing `set`, and the trace kernel DSL tries to track a gradient for the lookup twice

Really we don't even want to track gradients for number statement lookups! That would fix the error.