unlike other tools like iconv(1), a crop(1) without arguments or with
ones resulting in a no-op, like `-t 0 0', errors out. other options
like `-i 0' do not error. this breaks assumptions and results in
tedious intermediary steps or hacks like:
foo | {crop -t $1 $2 >[2]/null || cat} > baz.bit
instead, just ignore the check. subsequent code doesn't make
assumptions on that.
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