doc "Represents a file, link, or directory located at a 
     certain path, or the absence of a file or directory 
     at that path."
shared interface Resource 
        of ExistingResource|Nil {
    doc "The path of the file, link, or directory."
	shared formal Path path;
    doc "If this resource is a link, resolve the link."
    shared formal File|Directory|Nil linkedResource;

doc "A resource that actually exists—that is one that is
     not `Nil`."
shared interface ExistingResource 
        of File|Directory|Link
        satisfies Resource {
    doc "Delete this resource."
    shared formal Nil delete();
    doc "The principal name of the owner of the file."
    throws (NoSuchPrincipalException,
            "If set to a name for which there is no 
             corresponding user.")
    shared formal variable String owner;

doc "Thrown if there is no principal with the specified 
shared class NoSuchPrincipalException(name, Exception cause) 
        extends Exception(name, cause) {
    doc "The specified principal name."
    shared String name;