maskdata = function (matrix, reserved, mask) {
	var maskf = MASKFUNCS[mask];
	var n = matrix.length;
	for (var i = 0; i < n; ++i) {
		for (var j = 0; j < n; ++j) {
			if (!reserved[i][j])
				matrix[i][j] ^= maskf(i, j);
		}
	}
	return matrix;
}
Evaluates stack and returns value of a named property

@param {String} property - property name
@return property value or null if not found