Crash with difficult to decipher error messages

I ran a query on a 404Mb file which led to a crash of miller and the error messages were not very informative. Using ```head``` first didn't realist in an error, so I can only assume that the problem is due to some feature of the data later in the file. I haven't attached the file as it is large, but it's open data available here: [NSW Transport Open Data: Timetables Complete GTFS](https://opendata.transport.nsw.gov.au/dataset/timetables-complete-gtfs) I am using mlr 6.17.0 on macOS Tahoe 26.2 and installed miller using home brew. Command used: ``` mlr --csv step -a shift -f stop_id -g trip_id then step -a delta -f shape_dist_traveled -g trip_id then rename shape_dist_traveled_delta,dist,stop_id_shift,stop_id_prev stop_times.txt > distances.csv ``` Error messages: ``` fatal error: concurrent map read and map write goroutine 17 [running]: internal/runtime/maps.fatal({0x105023271?, 0x1055097c0?}) /opt/homebrew/Cellar/go/1.26.0/libexec/src/runtime/panic.go:1181 +0x20 github.com/johnkerl/miller/v6/pkg/mlrval.(*Mlrmap).findEntry(...) /private/tmp/miller-20260221-5490-ewif5u/miller-6.17.0/pkg/mlrval/mlrmap_accessors.go:195 github.com/johnkerl/miller/v6/pkg/mlrval.(*Mlrmap).Get(...) /private/tmp/miller-20260221-5490-ewif5u/miller-6.17.0/pkg/mlrval/mlrmap_accessors.go:22 github.com/johnkerl/miller/v6/pkg/transformers.(*tStepperShiftLag).process(0x8f64ff360a0, 0x8f6539e6a00) /private/tmp/miller-20260221-5490-ewif5u/miller-6.17.0/pkg/transformers/step.go:825 +0xa0 github.com/johnkerl/miller/v6/pkg/transformers.(*TransformerStep).handleRecord(0x8f5dc7f0d20, 0x8f653d0da90, 0x8f62d08a0a8) /private/tmp/miller-20260221-5490-ewif5u/miller-6.17.0/pkg/transformers/step.go:460 +0x53c github.com/johnkerl/miller/v6/pkg/transformers.(*TransformerStep).Transform(0x8f5dc7f0d20, 0x8f653d0da90, 0x8f62d08a0a8, 0x0?, 0x8f5dc6d8380) /private/tmp/miller-20260221-5490-ewif5u/miller-6.17.0/pkg/transformers/step.go:369 +0x78 github.com/johnkerl/miller/v6/pkg/transformers.runSingleTransformerBatch({0x8f62cc69908, 0x1f4, 0x0?}, {0x105554440, 0x8f5dc7f0d20}, 0x1, 0x8f5dc880000, 0x8f5dc884000, 0x8f5dc6d8380, 0x8f5dc7ec700) /private/tmp/miller-20260221-5490-ewif5u/miller-6.17.0/pkg/transformers/aaa_chain_transformer.go:260 +0x244 github.com/johnkerl/miller/v6/pkg/transformers.runSingleTransformer({0x105554440, 0x8f5dc7f0d20}, 0x1, 0x8f5dc802d20, 0x8f5dc880000, 0x8f5dc884000, 0x8f5dc6d8380, 0x8f5dc7ec700) /private/tmp/miller-20260221-5490-ewif5u/miller-6.17.0/pkg/transformers/aaa_chain_transformer.go:205 +0x7c created by github.com/johnkerl/miller/v6/pkg/transformers.ChainTransformer in goroutine 8 /private/tmp/miller-20260221-5490-ewif5u/miller-6.17.0/pkg/transformers/aaa_chain_transformer.go:180 +0x218 ```