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Let's say you have a GRIB message M1 with 5 keys in its MARS namespace: stream, step, levtype, levelist and param. And another message M2 has 4 MARS keys: stream, step, levtype, param.
grib_ls works by taking the keys in the namespace of the first message and uses those same keys for all subsequent messages. So it will display those keys in the first row of its output.
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