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Note that fieldsets are in fact using temporary disk storage on the host where MARS is executed, usually in $TMPDIR directory. When dealing with big fieldsets, you are advised to make such variable point to a suitable directory were where temporary data can be stored (e.g. $SCRATCH on ecgate/HPC).
One can extract fields (sub fieldsets) from a fieldset using square brackets. If X is a fieldset, X[2] is the second field, X[1,10] comprises the first 10 fields, and X[2,8,2] comprises every second field of x starting from 2 up to 8.
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