code: func (s *Scanner) scanExponent(ch rune) rune {
  	if ch == 'e' || ch == 'E' {
  		ch = s.next()
  		if ch == '-' || ch == '+' {
  			ch = s.next()
  		}
  		ch = s.scanMantissa(ch)
  	}
  	return ch
  }
docs: // scanExponent scans the remaining parts of an exponent after the 'e' or 'E'
  // rune.