code: @Override
    public Boolean decode(final String s) {
      checkNotNull(s);
      if ("true".equals(s)) {
        return Boolean.TRUE;
      } else if ("false".equals(s)) {
        return Boolean.FALSE;
      } else {
        throw new ConversionException(
            String.format("Strictly-converted booleans must be either 'true' or 'false', not %s", s));
      }
    }
docs: If <code>s</code> is "true" returns <code>true</code>; if "false", returns <code>false</code>;
  otherwise, throws a ValidationException.
  
  @param s May not be null.