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.