using the trigraph operator to select the lvalue is uncommon (because I've never used ALGOL))

As far as I know, ALGOL didn't have the trigraph operator; it actually has a way cooler way of selecting an lvalue - lvalues could be the return value of an IF statement. So, you could do something like:

    IF   var1
    THEN var2;
    ELSE var3;
    FI   :=   
    IF   var4
    THEN var5;
    ELSE var6;
    FI;
[download]

I can't remember the details of ALGOL syntax, so it could very well be the above code is missing semi-colons or parenthesis somewhere.

Abigail