code: function loadCached(options, callback) {
      var base = options.base + '-' + options.size + (options.retina ? '@2x' : ''),
          size = options.size,
          symbol;
  
      if (options.symbol) {
          symbol = options.symbol + '-' + options.size + (options.retina ? '@2x' : '');
      }
  
      if (!base || !size) {
          return callback(errcode('Marker is invalid because it lacks base or size.', 'EINVALID'));
      }
  
      if (!markerCache.base[base]) {
          return callback(errcode('Marker base "' + options.base + '" is invalid.', 'EINVALID'));
      }
  
      if (symbol && !markerCache.symbol[symbol]) {
          return callback(errcode('Marker symbol "' + options.symbol + '" is invalid.', 'EINVALID'));
      }
  
      // Base marker gets tint applied.
      var parts = [{
          buffer: markerCache.base[base],
          tint: options.parsedTint
      }];
  
      // If symbol is present, find correct offset (varies by marker size).
      if (symbol) {
          parts.push(xtend({
              buffer: markerCache.symbol[symbol],
              tint: blend.parseTintString('0x0;0x0;1.4x0'),
          }, offsets[size + (options.retina ? '@2x' : '')]));
      }
  
      // Add mask layer.
      parts.push({ buffer:markerCache.mask[base] });
  
      // Extract width and height from the IHDR. The IHDR chunk must appear
      // first, so the location is always fixed.
      var width = markerCache.base[base].readUInt32BE(16),
          height = markerCache.base[base].readUInt32BE(20);
  
      // Combine base, (optional) symbol, to supply the final marker.
      blend(parts, {
          format: 'png',
          quality: 256,
          width: width,
          height: height
      }, function(err, data) {
          if (err) return callback(err);
          return callback(null,  data);
      });
  }
docs: Load & generate a cached [a-z0-9] marker.
  
  @param {object} options
  @param {function} callback