code: function isPlainSafe(c) {
    // Uses a subset of nb-char - c-flow-indicator - ":" - "#"
    // where nb-char ::= c-printable - b-char - c-byte-order-mark.
    return isPrintable(c) && c !== 0xFEFF
      // - c-flow-indicator
      && c !== CHAR_COMMA
      && c !== CHAR_LEFT_SQUARE_BRACKET
      && c !== CHAR_RIGHT_SQUARE_BRACKET
      && c !== CHAR_LEFT_CURLY_BRACKET
      && c !== CHAR_RIGHT_CURLY_BRACKET
      // - ":" - "#"
      && c !== CHAR_COLON
      && c !== CHAR_SHARP;
  }
docs: Simplified test for values allowed after the first character in plain style.