code: function quotedPair() {
          var qp = wrap('quoted-pair',
          or(
              and(literal('\\'), or(vchar, wsp)),
              obsQP
          )());
          if (qp === null) { return null; }
          // a quoted pair will be two characters, and the "\" character
          // should be semantically "invisible" (RFC 5322 3.2.1)
          qp.semantic = qp.semantic[1];
          return qp;
      }
docs: email productions (RFC 5322) http://tools.ietf.org/html/rfc5322 3.2.1. Quoted characters quoted-pair     =   ("\" (VCHAR / WSP)) / obs-qp