ed P

A listing of procedure P is output to a temporary file.  The text editor is
the  called to allow the user to change the definition.  When the editor is
terminated, the file is read in causing the procedure to be redefined if no
syntax errors were found.  If the interpreter discovered syntax errors, the
user is asked if the procedure is to be re-edited.  If the response is 'n',
the old definition is restored.  Any other response causes the editor to be
invoked again.