Keep or drop symbols
Object of class
Specification of symbols to either keep (yes) or discard (no), coerced to a free object
keep() needs an explicit
return() to prevent it
from returning invisibly.
The functions are vectorised in the first argument but not the second.
The second argument—the symbols to keep or discard—is formally a
vector of nonnegative integers, but the functions coerce it to a free
object. The symbols kept or dropped are the union of the symbols in
the elements of the vector. Function
discard() was formerly
drop() but this conflicted with
These functions have nothing in common with APL's
Robin K. S. Hankin
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