Non-standard extensions to Lisp 1.5 to read and write to the filesystem.

Lisp 1.5 had only READ, which read one S-Expression at a time, and various forms of PRIN* functions, which printed to the line printer. There was also PUNCH, which wrote to a card punch. It does not seem that there was any concept of an interactive terminal.

See Appendix E, OVERLORD - THE MONITOR, and Appendix F, LISP INPUT AND OUTPUT.

For our purposes, to save the current state of the Lisp system it should be sufficient to print the current contents of the oblist to file; and to restore a previous state from file, to overwrite the contents of the oblist with data from that file.

Hence functions SYSOUT and SYSIN, which do just that.