For any pair of object language lists of the form --`[x1;...;xn]`-- and
--`[y1;...;ym]`--, the result of evaluating
APPEND_CONV (--`APPEND [x1;...;xn] [y1;...;ym]`--)
is the theorem
|- APPEND [x1;...;xn] [y1;...;ym] = [x1;...;xn;y1;...;ym]
The length of either list (or both) may be 0.