A common source of bewilderment on this system is that although you are communicating with the same computer all the time, you are not talking to the same program all the time. Beginners often talk about ``the computer'', as though it were one indivisible entity. It is more accurate to think in terms of multiple personalities, each program having its own personality, memory, and language, and knowing nothing of the others sharing the same machine.
So although the operating system and Prolog both share the VAX, they follow completely different conventions. Prolog needs sentence terminators; the operating system does not. Beginners will often type
eden.to the operating system, and get understandably confused when it complains about an unwanted dot. Similarly, some people forget to start up Prolog after logging in, and try typing Prolog commands at the operating system, provoking cascades of error messages. To get round this, the best advice is: watch the prompt! Prolog's prompt is
?-; the operating system's is always $.
The networks and intermediate stages are another source of bewilderment.
When you start up the terminals in CTC, you begin by talking to a
program running on the micro-computer there. The commands you give that
tell it to connect to the University master switchboard. In turn, you
must tell the switchboard to connect to the VAX. Then you tell a VAX
LOGIN program to log you in, typing your username and password; then you
tell the operating system to start up Prolog; and finally you talk to
Prolog. So you have all the sub-systems shown in
figure 
to deal with.
Micro <-> switchboard <-> LOGIN <-> VAX operating <-> Prolog
system
^
______|_______
|You are here|
--------------
Figure 
Subsystems on the VAX.
It is difficult at first to remember which sub-system you are talking to at any stage, but practice makes it all seem easy, eventually.