code: func lexLeftDelim(l *lexer) stateFn {
  	l.next() // read the first {
  	// check the next character to see if it's a double delimiter
  	if r := l.next(); r == '{' {
  		l.doubleDelim = true
  	} else {
  		l.backup()
  		l.doubleDelim = false
  	}
  	l.emit(itemLeftDelim)
  	return lexBeginTag
  }
docs: // lexLeftDelim scans the left template tag delimiter
  //
  // If there are brace characters within a template tag, double braces must
  // be used, so we differentiate them to match double closing braces later.
  // Double braces are also optional for other cases.