Consider

restart;
L := [1,2,3,4];
x[j];
seq(%, x[j] in L);
x[j];

What would you expect the output of the last line to be?

Answer:   Maple says 4.   Why?

I want to write a proc, say f, that takes an single argument Z, as in
f := proc(Z::?) ...
where the only acceptable Z values are pairs [x,y], where x and y are selected from the set {a,b,c,d}. The entries a,b,c,d of that set are prescribed symbolic constants. 

Thus, the following are legal calls to f:
f([a,a]), f([a,b]), f([d,c])
but these are illegal:
f([a]), f([a,b,c]), f([a,x])

I don't know what to put for the "::?" in the proc's definition in order to enforce those constraints.  Any ideas?

Extra: In fact, I would prefer to ban the f([a,a]) option as well (no repeated symbols) but that's of secondary importance since it is easy to inspect and handle that within the proc's body.

Before Maple 2020, it was possible to start up Maple from the Linux commandline with multiple files, as  in

maple   -x   worksheet-1.mw   worksheet-2.mw   ...   worksheet-n.mw

That started up Maple's Java interface and loaded the n worksheets in separate tabs.

That no longer seems to work in Maple 2020 — now the command exits silently when invoked with multuple files.  Why?

Aside: Loading a single file works as expected, but not multiple files.