code: function tokenize(state, silent) {
  	if (silent) {
  		return false;
  	}
  
  	const start = state.pos;
  	const max = state.posMax;
  	let momChar = state.src.charAt(start);
  	let nextChar = state.src.charAt(start + 1);
  
  	// we're looking for two times the open symbol.
  	if (momChar !== MARKER_OPEN || nextChar !== MARKER_OPEN) {
  		return false;
  	}
  
  	// find the end sequence
  	let end = -1;
  	nextChar = state.src.charAt(start + 2);
  	for (let i = start + 2; i < max && end === -1; i++) {
  		momChar = nextChar;
  		nextChar = state.src.charAt(i+1);
  		if (momChar === MARKER_CLOSE && nextChar === MARKER_CLOSE) {
  			// found the end!
  			end = i;
  		}
  		if (momChar === MARKER_OPEN && momChar === MARKER_OPEN) {
  			// found another opening sequence before the end. Thus, ignore ours!
  			return false;
  		}
  		if (momChar === '\n') {
  			// found end of line before the end sequence. Thus, ignore our start sequence!
  			return false;
  		}
  	}
  
  	// input ended before closing sequence
  	if (end === -1) {
  		return false;
  	}
  
  	// start tag
  	state.push('kbd_open', TAG, 1);
  	// parse inner
  	state.pos += 2;
  	state.posMax = end;
  	state.md.inline.tokenize(state);
  	state.pos = end + 2;
  	state.posMax = max;
  	// end tag
  	state.push('kbd_close', TAG, -1);
  
  	return true;
  }
docs: /*
  Add delimiters for double occurrences of MARKER_SYMBOL.