- uniq() : creates an IndirectList with duplicated entries
filtered out
- subset() : creates an IndirectList with positions that satisfy
a condition predicate.
- subset_if() : creates an IndirectList with values that satisfy a
given predicate.
An indirect subset will be cheaper than creating a subset copy
of the original data, and also allows modification.
STYLE: combine UIndirectList.H into UIndirectList.H (reduce file clutter)
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