Multi-level operations

The more sophisticated use of F-returns was to make the original call with a multi level operation. This was an operation with several possible actions. If the first returned with F-return, the second was automatically tried. The major user of this feature was to be the Disk system. It would provide a two level operation, in which the first level was an ECS file read or write, and the second was a call on the Disk system subprocess. Thus, a user could attempt to read the ECS version of a disk file with this operation. If the desired portion of the file was in ECS, the action would proceed exactly as if it were an ordinary ECS action (i.e., fast). If not, the ECS system would F-return, and an automatic call would be made on the Disk system to handle the situation. This would be an inherently slow subprocess call. Thus, an initial expensive call on the Disk system was avoided if the data were actually in ECS, but it all appeared as one simple action to the user.