# If <argument> is blank or another option, print an error message and  exit
# with status 1.
_require_argument() {
  # Set local variables from arguments.
  #
  # NOTE: 'local' is a non-POSIX bash feature and keeps the variable local to
  # the block of code, as defined by curly braces. It's easiest to just think
  # of them as local to a function.
  local _option="${1:-}"
  local _argument="${2:-}"
