op NilTests( i:integer, l:integer*, t:integer and integer, s:string) = (

    -- The following 4 exprs are all OK, since nil is compatible with any type
    i = nil;
    l = nil;
    t = nil;
    s = nil;

    -- The following 2 exprs are OK, since [] and {} are alternative
    -- designations for nil
    l = [];
    t = {};

    -- The following 3 exprs are all OK as well, since [] and {} are just
    -- considered to be alternative "spellings" of nil.  Strictly speaking, I'd
    -- call this a weakness of the current type system, but let's just leave it
    -- as is.
    l = {};
    i = [];
    s = {};

    -- The following 2 exprs have type conflicts, for the indicated reasons.
    -- Of particular note 
    l = 0;	-- lists and ints are not compatible
    l = "";	-- lists and strings are not compatible

    let l = [1,2,3];
    let i = l[4];

);