define method trim
    (sequence :: <sequence>, remove? :: <function>, #key from = #"both")
 => (sequence :: <sequence>)
  let keep? = complement(remove?);
  let start-index = member?(from, #[left:, both:]) & find-key(sequence, keep?);
  let end-index = member?(from, #[right:, both:]) & find-last-key(sequence, keep?);
  let end-index = if (end-index) end-index + 1 end; // Still #f if end-index is #f.
  if (~start-index & ~end-index)
    sequence
  else
    copy-sequence(sequence, start: start-index | 0, end: end-index | sequence.size)
  end
end method trim;