code: function ( qu, value ) {
  		return (sys.isNull( value ) || sys.isUndefined( value )) !== qu.operands[0];
  	}
docs: `$exists` Sees if a field exists.
  `{field: {$exists: true|false}}`
  @name $exists
  @memberOf module:documents/probe.queryOperators
  @example
  var probe = require("documents/probe");
  probe.find( data, {"name.middle" : {$exists : true}} );