ed &optional x => implementation-dependent
Arguments and Values:
x---nil, a pathname, a string, or a function name. The default is nil.
ed invokes the editor if the implementation provides a resident editor.
If x is nil, the editor is entered. If the editor had been previously entered, its prior state is resumed, if possible.
If x is a pathname or string, it is taken as the pathname designator for a file to be edited.
If x is a function name, the text of its definition is edited. The means by which the function text is obtained is implementation-defined.
Affected By: None.
The consequences are undefined if the implementation does not provide a resident editor.
Might signal type-error if its argument is supplied but is not a symbol, a pathname, or nil.
If a failure occurs when performing some operation on the file system while attempting to edit a file, an error of type file-error is signaled.
An error of type file-error might be signaled if x is a designator for a wild pathname.
Implementation-dependent additional conditions might be signaled as well.
pathname, logical-pathname, compile-file, load, Section 19.1.2 (Pathnames as Filenames)