code: function(key, val) {
  				var self = this;
  				return this.done(function(s) {
  					if (!key)
  						return new Error(".toState need key/val couple.");
  					val = (typeof val === 'undefined') ? s : val;
  					self._state[key] = val;
  					return val;
  				});
  			}
docs: set key/value in current state
  
  @chainable
  @method context
  @param  {String} key
  @param  {*} value
  @return {deep.NodesChain} this