also, i find myself not using clos or defstruct all that much in my cl code. i tend to use ordinary data structures like lists and arrays.
my recent example (an excerpt from which i already showed) was a sudoku solver. i wrote one version in CL, then thought i'd do it in arc for fun. in CL, the puzzles were simply arrays (in Arc, they were lists do to the lack of an array type).
However, i didn't want my code to use the array (or list) accessors built into the language to access values in the puzzle, because that is not abstract enough. so i defined a function called pval that takes the puzzle and an x and y, giving the value in that square. that way, my code deals with puzzles, not arrays.
Then I wanted to be able to set the value in a square using the same pval accessor. Trivial in CL, but Arc made it a little difficult. This comes up rather often for me.
That point might not have justified such a long explanation. Whoops.
