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Specified with only the fieldset as its single argument, sort() sorts in descending order the fieldset according to date, time, step, (ensemble) number, level and parameter paramId.
The second argument allows you to modify the precedence of the sorting keys - e.g. if the second argument is a string "parameter", then the sorting is done according to the parameter first. If the second argument is a list you specify a list of sorting keys - e.g. ["parameter", "date"] sorts on parameter and then date.
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