code: function _bindSingle(combination, callback, action, sequenceName, level) {
  
              // store a direct mapped reference for use with Mousetrap.trigger
              self._directMap[combination + ':' + action] = callback;
  
              // make sure multiple spaces in a row become a single space
              combination = combination.replace(/\s+/g, ' ');
  
              var sequence = combination.split(' ');
              var info;
  
              // if this pattern is a sequence of keys then run through this method
              // to reprocess each pattern one key at a time
              if (sequence.length > 1) {
                  _bindSequence(combination, sequence, callback, action);
                  return;
              }
  
              info = _getKeyInfo(combination, action);
  
              // make sure to initialize array if this is the first time
              // a callback is added for this key
              self._callbacks[info.key] = self._callbacks[info.key] || [];
  
              // remove an existing match if there is one
              _getMatches(info.key, info.modifiers, {type: info.action}, sequenceName, combination, level);
  
              // add this call back to the array
              // if it is a sequence put it at the beginning
              // if not put it at the end
              //
              // this is important because the way these are processed expects
              // the sequence ones to come first
              self._callbacks[info.key][sequenceName ? 'unshift' : 'push']({
                  callback: callback,
                  modifiers: info.modifiers,
                  action: info.action,
                  seq: sequenceName,
                  level: level,
                  combo: combination
              });
          }
docs: binds a single keyboard combination
  
  @param {string} combination
  @param {Function} callback
  @param {string=} action
  @param {string=} sequenceName - name of sequence if part of sequence
  @param {number=} level - what part of the sequence the command is
  @returns void