code: func (s *Scanner) scanMantissa(ch rune) rune {
  	scanned := false
  	for isDecimal(ch) {
  		ch = s.next()
  		scanned = true
  	}
  
  	if scanned && ch != eof {
  		s.unread()
  	}
  	return ch
  }
docs: // scanMantissa scans the mantissa beginning from the rune. It returns the next
  // non decimal rune. It's used to determine wheter it's a fraction or exponent.