code: function isPlainSafeFirst(c) {
    // Uses a subset of ns-char - c-indicator
    // where ns-char = nb-char - s-white.
    return isPrintable(c) && c !== 0xFEFF
      && !isWhitespace(c) // - s-white
      // - (c-indicator ::=
      // “-” | “?” | “:” | “,” | “[” | “]” | “{” | “}”
      && c !== CHAR_MINUS
      && c !== CHAR_QUESTION
      && c !== CHAR_COLON
      && 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_SHARP
      && c !== CHAR_AMPERSAND
      && c !== CHAR_ASTERISK
      && c !== CHAR_EXCLAMATION
      && c !== CHAR_VERTICAL_LINE
      && c !== CHAR_GREATER_THAN
      && c !== CHAR_SINGLE_QUOTE
      && c !== CHAR_DOUBLE_QUOTE
      // | “%” | “@” | “`”)
      && c !== CHAR_PERCENT
      && c !== CHAR_COMMERCIAL_AT
      && c !== CHAR_GRAVE_ACCENT;
  }
docs: Simplified test for values allowed as the first character in plain style.